Can a 1975 dt100 be timed?  I see a few things in the internet that it can..... but I dont see any timing marks on the flywheel,  there are no elongated screw holes on the stator to allow to adjust left and right and I dont see a timing adjustment screw around the ponits.... 

Timing is just set or altered by points gap. Tight 15 thou should be in the ball park of yamahas recommendations. Think your model has a central plug so could use a dial gauge to find 1.8mm btdc but i wouldn't worry too much unless you have a reason to worry, like points heal is worn down or points pas are burnt away. I just like it keep it simple if pos.

Look in the tech library, link up top of page, for a service manual for your bike. It will have instructions on how to time it. Lot's of other good info in the tech library too.