I also agree on a stock piston. I cut my pistons about a quarter inch but I also had a GYT kit head and pipe and a 30mm carb. We did everything then by the seat of the pants, there was no internet!

Rebuilding and reassembly of the engine has been completed, the only surprises found inside were the altered piston skirts (replaced with a stock Yamaha 0.25 oversized piston after cylinder was bored and honed), and the circlip on the kick start idler gear had come off which allowed the gear to move around and create excess wear. The gear and related parts were replaced, all appears well now.
Of course, after finishing this assembly, I sourced an MX cylinder head on eBay, so now waiting for it to arrive. I guess putting the original head on was just for "practice"...

Looking good!!

You stole that baby! The frame lowering kit will make the bike handle amazing off road (at least compared to a stocker!) I have ridden Dave Meis’ 360 With that mod and would love to do it to one of mine.

You may be able to sell one of those aluminum fenders for a huge percentage return on your original investment as well!

I would actually consider keeping the “patina ” and just do mechanical updates. I like the Mad Max look that one has. It sort of looks like a time machine. Call it the Delorean . . .

The idea of the welded bar is as pointed out if you get stuck in deep doo doo it's an extra advantage point for lifting up there's a few bikes I've seen have similar attachments.as for the carb it does help with excellaration my ex father in law used to race 125 cc bantams years ago and they were always tweaking.
Nice find though ,bargain ,but I too would recommend going back to stock bantams back then did have a habit of blowing up and overheating when you they started adjusting and tweaking.
Dave

The idea of the welded bar is as pointed out if you get stuck in deep doo doo it's an extra advantage point for lifting up

I wouldn't have thought of that, but makes sense. Maybe it is worth keeping since this bike isn't going to be original anyway.

Don't shorten the piston on the exhaust side or you'll uncover the exhaust port at TDC. I'll go measure the difference between DT1 & MX shortly.
4mm shorter on DT1MX. I see its on both side, ex & inlet, but i'd be careful going any shorter on the exhaust side but easy to mark it in the ex port at tdc & see how far you can go but no advantage in shortening that side. I've gone pretty radical on inlet side experimenting with smaller motors when we were bucket racing miniature "road racing" on farm bike based 125 two strokes. One cylinder i had no time to modify enough before the event & have a piston with eleven mm of the inlet side. Mind you it doesn't look like it got much use surprisingly lol. Probably just shortened it until a had about 200 degrees in let timing but the port was only standard so must've got away with it to a point. Would've been a TF Mudbug Cylinder not TS125ER but same cylinder with small ports. TF100 not 125--125 came later sorry when the rules were opened up to restricted 125's. but 100's were anything goes.

I too modified a DT1 piston, I don't think it was 7.5mm, 3mm comes to mind. I read about it somewhere...don't remember if it helped, got it free from buddy, a complete novice. I watched him in horror as he approached a 3 or 4' ditch at 25 or so, & the endo he did! I thought suspension was for soaking up bumps he said! A few weeks later @ Carnagee (spelling wrong) in Livermore,Ca we went for a "warm up ride" & returned to trucks to fiddle, & we didn't see Keith ALL DAY & he finally comes back, he rode down through the trees to a ravine & was all day getting back out! We all laughed until we couldn't stand! On the two hour ride back I told him he'd better get rid of bike before the next foolish thing he did might kill him, he gave it to me! I already had a 360mx & didn't need it is why it's piston modified b-4 selling.

Beginning the reassembly process after a couple months of minimal activity, everything now powder coated or zinc plated, all new hardware and chrome bits being installed. The frame had a Webco 2" lowering kit welded in, I left it that way. The swing arm is a 1" extended aftermarket part. A new front end has been adapted from a 1979 XT500 with 36mm fork tubes and about 2" more travel than the original DT1 parts. The rear shocks are 13" Girlings.
I plan to use aluminum rims, 21" front and 18" rear, and a high mounted front fender. All the original lighting will go back on to keep it street legal.
A ways to go, but making progress...

Looking good! I like your mods and approach to the rebuild. Can't wait to see more!