A Suzuki PE 175 dirtbike motor will fit, and i've heard the motor nearly bolts right in! It is an air cooled motor though, but way more HP and snap than a stocker 200x. I dont know what all has to be done in order to make it work, but I remember years ago on here a guy did it, and it ripped! There should be a vid on youtube, I'll have to see if I can find it. ROOTS!!
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