Yup...This is something I am considering.....My brother has a 1986 Suzuki GS650 4 cylinder bike. He really wants to take the engine out of it and shove it into my beartracker.....it would fit perfectly with the shaft drive (so he says)...I'll post if we ever get around to doing it put its something we may do...man.....that thing would be liek too fast.

 

Cool project.
I've got a GS550le bike. I think it's a le or something like that anyways it's the 550 4 cylinder motor in it and i'm going to build it into a golf cart then lower the cart a bit so it's not as tippy. should be a sick machine when i'm done.
Good luck with your bike to quad rebuild project. Sounds like fun.

 

That 650 motor probably dosent make a whole lot of HP but it will have a good bit of torque.

 

I once put a 1000cc yamaha virago V-twin in a 225 moto-4. Wasn't worth the time and money but it looked cool.