Where can i find parts to a 2 stroke 100cc yamaha 1986 model YAM3UL0097

That is the model number for a 1990's Yamaha RT 100A - 100cc Dirt Bike.

https://www.google.com/search?q=YAM3UL0097

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Are you able to measure the bore and stroke? Cubic centimeters displacement would be pi times radius squared times height.? The dealer can take those numbers off the engine and do the job,why not? Match up the old piston if you can find it

I just ran the VIN on the Yamaha dealer system and it came back as invalid. Can you double check it?

I just ran the VIN on the Yamaha dealer system & it's not valid. If you can you post the correct VIN on the frame I'll run it again but that one's not valid according to Yamaha.

The bike was only made for two years 1986-87. Most if not all parts are interchangeable. There was also a 700 made for the US over the same years and those parts are mostly compatible as well.

Quote your frame number to your local Yamaha dealer they will have the search data you need

On a machine as old as this any present-day 2-stroke oil will outperform the original manufacturers specification.

Older models have the build date and model on the steering head even a 73 will have the build date and the VIN and the country it's built in. not knowing what era your bikes in all *** bikes have the CC of the engine on the front of the block under the fins near the base of the cyclinder 249 cc, 348cc 174 cc 123cc, 99cc, 49cc If you tell me what cyclinders it has does it have an extenal oil tank does it have numbers anywhere like YM1 YDS3 . An AS1 would be a 100cc twin 2 stroke with an extenal oil tank and upsweeped pipes.

The site below (although European based) has all the colour reference codes applicable. Yamaha are a parts bin manufacturer, in that they cobble necessary parts from different models and sometimes share a range of colours across a model, particularly fashion type schemes.
The second link is bikepics, make a consensus from all the models shown for your bike.
Cheers.

http://www.rsbikepaint.com/

http://www.bikepics.com/yamaha/rd350/

Congrats on the purchase! easy way to tell look at the exhaust pipe. if it is just a thin tube that is tight to the engine and goes right to the muffler it is a 4 stroke (250f). the four strokes have a square-ish head. Deep/bass-y engine sound. If you have a large (probably black) exhaust pipe with a large bulge in it (5-6" diameter) that wraps around the front of the frame, its a 2-stroke. They have a more round cylinder head. Beautiful pingy sound and a little smokey at start up. DON'T FORGET TO MIX YOUR GAS!!! there should be a serial number sticker at the very front of the frame (behind the number plate where your triple clamp/handlebars connect) Probably on the left side. Hope it helps! rate please :)