I think your best bet would be to sell your quad and look for a different one. Since you are dealing with a 2wd, shaft driven machine, your choices of a direct swap are probably limited to the motors I mentioned in my previous post. A motor out of a newer Foreman 500 2wd may be doable, but it would take work and would probably not be worth the time, effort and money. Not only do you need the motor it there, but you need all the electrical, the ignition system and the intake, carb and exhuast systems to fit in there as well. A motor swap is more difficult and expensive than most people realize. Things need to be moved, mounts need to be fabricated, and improvisations need to be made. You would really be best just getting a different quad that better suits your needs.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.