Speedo needs rechroming and the tach just a little.....Missing reset knob... Ya you are right some black rubber might distract from the not so pretty chrome

Where can the rubber rings for the tach and speedometer be found ??????  I guess the speedometer shares the same with many others but NOT that tach

Yep, true with the rust but i was just think they look 'so neat' with the big & small instruments & rubbers. Might have to get two big rubbers then shorten & glue one.

Regarding the Fork Caps .
Looking at my 68 DT1 parts manual ( late 68 print date ) , it shows two options .
The vented caps were the superceded version , and were used on 68's through 69 model run .
There is no detail on exactly when in the production run , the change was made .
Seeing as your bike is a  very low production number , your caps could very well be correct ,
as I'd suspect , they wouldn't have changed things quite that early in the peace .
Quite possibly yours may have been in the first production run .
 

I just checked eBay and this guy has three right now.

Xttps://www.ebay.com/itm/115409807155?hash=item1adef5e733:g:VFEAAOxy4kpQ9cQz&fits=Make%3AYamaha%7CModel%3ADT1%7CYear%3A1968%7CSubmodel%3A250

 

I seem to recall seeing a Tacho rubber listed with one of the repro companies ,
Possibly Clauss Studios .

Thanks Tom P

Here is Clauss Studio

www.claussstudios.com/store/p651/Yamaha_...Trim_Ring_1968_.html

Deet if you see this,,, Is this the correct one ????

Clauss Studios has the small tach ring.  www.claussstudios.com/store/p651/Yamaha_...Trim_Ring_1968_.html
Someone already posted speedo damper that's on ebay. (Note: 68 dt1 gauge dampers were slightly different - they didnt have a beveled bottom, but hensteeth to find). 
Horn upside down.  Front rimlock has been replaced, rear rimlocks are correct.  Fuel tank missing rear rubber fitting band
Regarding head, I agree it does look like GYT head but there was a mod back in the day (just my opinion based on I've seen 4-5 like yours) where the OEM single plug head was very nicely modified for a center plug.  DT1-00490 head was modified identical to yours.  I believe you have the original head that was modified; as the GYT head left fin was different from enduro head, your left fin is an enduro fin IMHO.  I would post a photo of 00490 head but not sure which box it resides in the shop.  
Small item, seat strap bolts to oil tank bolt but is more functional where yours is mounted as seat will stay raised without holding up.
I'm in the camp of fork caps were vented.  Others have already commented on other items.
Nonetheless you have an outstanding early production bike with excellent patina!!
Congratulations!!