Well, it sounds to me like the motor has to come out of the quad anyway. I would take it out, and compare everything side by side and make you decision based on what you see. Alot of those Honda utility motors are identical, and they have been generally the same for years. You may even be able to swap parts inside the gearbox to make it match up with your quad. If your 400 motor is sitting there doing nothing, and the other one is junk, I don't see what you have to lose by just checking it out, and seeing how it will work. Good luck to you, whatever you decide.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.