Consider the feasibility.

Buy a 2500$ bike, to ditch the engine and put in a $$$ engine?
if a 650 single is what you want, then buy it and hardtail that...

been down this road

http://www.mychinamoto.com/forums/sh...ll=1#post69634

i bought a 2002 LS650 with 6500 miles in nice condition for 500$ , that is engine and frame...
there are hardtail kits, < 300$, you now have a complete bike for a fraction of the price...

either way you will need to know how to weld... so i cant see the the justification...