I'll tell you right now >>> Dont waste your time! They wont work, because the 83-84R seat comes down a bit in the middle, not a flat pan like 85-86. You would end up ruining your fenders to make it work. Still, if you did manage it, there would be gaps all over the place, between the seat and fender. Your best bet is to repair (plastic weld) your old fenders if there is enough there, or try to make a fiberglass mold out of them, or probably the easiers - buy the correct 83-84 fenders. I believe the 85-86 front fender would go on without issues. I have seen a lot of them used, especially on 200x's. 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