I'm computer challenged but i'll try to link some pics i took earlier but not the best.
Thought i had the KX125 filter but feeling in there it's a K&N & one pic shows a filter the same & those filters comes with a cap.
Just have a bit of flat acrosswith a peg into rubber in the cap & the flat has ears bent up & bolted through the side of the pot & has downward pressure on the filter to make it seal in the pot.
Shocks are probably out of class depending what year our rules stretch to.
Ok in Pre "78. Front forks look longer but are just ordinary old DT250 but from about "77 so not that long in the travel, just look long.
Rear i think are just the short travel--'short overall'--Gas Girlings with 4 inches travel if ya lucky but i arc welded them into PE250 bodies to get the overall length.
Angling them forward increases axle travel of course & there's rules on how far forward bottom & top you're allowed within certain classes.
I extended the axle lug so with the rules are too far forward at the bottom too.
I used to have it with leading axle forks & alloy arm which was ok but a very long wheel base.
Only changed it back to have it fit more into a suitable class. But Then i woke up haha, so not much has happened since.
Doesn't matter i'm pretty much past it anyway. Just a silly old fart lol.
When i get inspired i have a 280cc motor [250 DT1MX top end on a mixture of DT1 cases & bits but using a 360 70mm stroke crank--so 70x70 instead of 70x64 bore & stroke] to try.
I did run it years ago & went quick down the road & whatever ratios i sorted they felt better than the ones in the 360 which are terrible, or normal terrible for the trail bike & useless when all the power is high up & little torque where you need it for wide ratio spacing.
Need another frame then i could build another complete rat bike. But frames are pretty scarce here for "68[250] to "73 250-360.
Just plucked a bit out of our rules i found online if its current.
Forgot my link for the filter setup. Hope it works.
photos.app.goo.gl/mnKGDo36o4ndgJhC6

Classic (Circa 74) - Small Bore, Mid Bore, Big Bore

1970 - pre 1975.
Max 4” rear suspension travel, max 7” Front suspension travel.
Twin shock or cantilever suspension, drum brakes and air cooled only.

Pre 78 - Small Bore, Mid Bore, Big Bore

1975 – pre 1978.
Max 9" rear and front suspension travel.
Bikes must look like they were available no later than 1978.
Twin shock or Cantilever suspension, drum brakes and air cooled.
If water cooled - the system must be period e.g. Fox and Shiobi water cooled heads.

I see i put a link back a page to when i had long legs on it.
You can just see the blue short Gas Girling in the pic welded into PE bodies.
I'll bring that link to here just to make it easier. Was an RT1 top end them which i'm kicking myself for selling!!.
Can't find it but don't remember selling.
photos.app.goo.gl/yHo7eYmCyuB9TAhU6

I love it. Great creative thinking and experimentation. 

70x70 motor is cool. Love it 

So the back of the pot is just open and then I think I see kind of a mudflap to cover up the back and stop the roost from the tire? 

I had considered a modern yz250 crank for my mutt of an rt3. 72mm stroke easy to come by and I think it might open up more modern ignition options. I think the modern cranck uses oddball 28mm main bearings but pro x makes a 28x72 bearing that works probably fit. I have no idea about all of the rest of the details and the reality is it's fun to think about but probably I'll never do it I'll just get this old things running and have some fun with it hopefully. 

What's the pipe in the long travel setup? I kinda like how it tucks out of the way.

Pipe in the long travel pic is an RM250 pro-circuit pipe I modified to fit about 30 years ago. Was ok with a std cylinder but with RT2mx cylinder it was useless with light switch power starting way up high & revving to the moon with no torque. I mite have a pic of the right side showing it.
Found a right side pic showing the pipe.
Fuel tank is an AT1 which looks the same but smaller than DT1.
Pipe probably would have hit up under the DT1 tank.
Shortened sub frame of course.
Mudflap in the pics as it is 'now' sits closer to the filter box than this pic.
I did plan [never happened] to try just stretching a filter right over the top as a quick change filter if it would pass enough air, to save the drama of getting at the main filter during the day of racing.
Originaly about 30 years ago i had it setup good with a homemade pipe i welded up using info & calculations from a book i had & it worked perfect.
Had a PE250 swinging arm in then which was a bit shorter than this one & looked right.
Had a new RT2 silver grey tank too.
Then i sold it all.
Then 20 years later the guy gave me the bare frame back but to S-arm.
So i started again lol shuffling boxes of bits around.
Wish i'd kept that RT1 piston port cylinder but would be good to have an RT1MX cylinder.
When my Boss had a new RT1MX in "70 i borrowed his motor [while he was overseas haha] & ran it in my DT1 i was motocrossing at the time.
Was good too--then he came home!!.
photos.app.goo.gl/qfzQxFR5ErjYutTB8

Hey RT325 figured this is probably another question you would have some insight in. 

I was looking through parts lists and specs thinking about a close ratio transmission. The obvious answer would be a -mx gear set to swap in but then I noticed it looks like the early dt1's, 69-70, had close ratio transmissions, actually a little closer ratio than the mx's, and probably less rare/ cheaper. 

It's that right? Some of the documentation is a little sketch so I might have it wrong but seems to be the case 

Any reason not to go that way? Like did they not fit or have some weakness or issue that is best avoided? 

 

Back in "68 i didn't know there was wide & close ratios on DT1 & we just raced what we had.
But i always liked the ratio spacing on mine except i broke the spindly 3rd year big wheel.
On those boxes you could swap the sliding gear from one shaft to the other when the dogs got "dog-eared" & then they are spinning the opposite way giving the good edge of the dogs which got ya going until next time.
When i broke mine it came out the bottom of the case.
But in much later years of gathering parts i made up a coarse tooth box for my 360 which was good until it broke 'that' gear also & made an even bigger mess.
Those early boxes ran the 4th 5th selector wrapped around the machined smooth drum.
So if you go that way with just shafts & gears with later drum & selectors you will need to look closely at the selectors & how they fit on the gears as i think they altered sizes.
Another thing that nearly caught me out was--my best drum on whatever i was mix n matching was the first machined drum but i was using later selectors not the wrap around.
So the middle groove is machined in reverse as wrap around has the pin over behind.
So on my bench practice shot i had 1-2-3-5-4 haha. Always things to catch us out.

Hmm, so maybe the early box is not a great option.