At least some of the BionX motors ahve their controller inside. If yours is like that, then to disconnect the motor from the controller (to verify if it's an electrical or mechanical problem) you'd have to unlace the wheel from the motor so you can open it up, then disconnnect the phase wires from the motor coils to the controller board inside. This is a last step, to be done only if you can't find any other problems, because unless you're experienced at wheelbuilding (lacing, tensioning, truing) then it's a learning process to put it back together. And also because since BionX is gone as of earlier this year, parts are going to be harder to find.
As far as replacing any parts go, you'd have to replace them with the original version and type, or the system probably won't work, because BionX is all proprietary and integrated--if anything stops working, the rest of it also doesn't work (in general--it's possible certain problems wouldn't disable the whole system, but many will).
If you dont' replace them with originals, you're left with having to hack the rest of the system to make it work, and that's going to be a process of trial and error, and some of the parts are simply not going to work without some of the other stuff working.
If the wheel uses rim brakes, then it's possible the brakes are rubbing on the rim from a misadustment of hte brakes or the rim. Or if the wheel is offcenter or damaged, it could be rubbing on the frame.
If it's a rear motor, then you could also take the chain off to ensure it's not a stuck freewheel/chain problem.