The most common clutch mod is from a late 70's DT 175, or MX 175. Im not sure, but only certain years will work, not all of them. You have use the mainshaft from the YT, the shaft the sprocket runs on. The mainshafts on the dirtbike DT and MX is shorter, and the chain will not line up from the rear sprocket to the front. I have been wanting to do this, but havnt been able to pick up a cheap DT, or MX motor or bike for cheap. I think it would be best to but a whole motor, or at least a bottom end, and go from there, then start transfering parts. Vealmonkey knows all the details to this swap. Good luck!
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