Some people have been using the DT 175 and MX 175 dirtbike motors, but you still have to swap some things from the 125 or 175 yt to the dirtbike engine to make it work. These DT and MX bikes are somewhat hard to find for the year you need (mid to late 70's) They are yamaha dirtbikes. Try a search, and you will find what I am talking about. When it comes to tearing the entire motor apart, a manual would always be a handy thing to have. Good luck. ---- Jon.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes