Once you have a new rear wheel, fit up the engine-side drive sprocket and install the new motor. Then see if you can run the chain from motor to wheel with a decent amount of clearance between your chainstay and the chain.
If it looks like your drive chain won't tear up your frame while you're riding, you are good to go. You can adjust your tension by moving the rear wheel further back in the dropouts. And if you need to adjust your pedal chain, you can put the tensioner over there on that side and not worry about the same kind of problems.
If, however, it looks like the lower part of the drive chain is going to try to saw through the chainstay - do not despair! Phantom Bikes makes this beauty: http://www.phantombikes13.com/images...ioner%20lg.jpg
It is a rather long tensioner bracket that spans the lower drive-side chainstay and the upper drive-side seatstay. Once you mount your chain guide wheel to this, it can't get pulled into your spokes. Only $35 for a fine piece of machining, and it should pay for itself in the wheels it saves.