my boss has an 86. it has a 650 ninja motor in it with wheelie bars and 24in sand paddles
loads of fun and scary fast, 60 in 2nd gear, great project but lots of mods to the frame to make it fit

 

i would either go with a sportbike engine like a 600 ect, or try getting a dual sport engine, like a xr650 would be nice, loads of power from top to bottom, if you like 2 strokes, get a sled engine you can find one for real cheap, try getting a artic cat twin 700 or 800 those haul some a$$, and they have no gears so your just off, off the line.