Be cheaper to buy a new engine for it, if indeed the crankshaft is bent. If you've bent your crankshaft it means you've probably done it by hitting something, which would have sent shockwaves throught the engine. You've basically knacked it.
Otherwise, if its just hitting the plastic at the back, then its your axle bearings that have gone. Simple cheap job. Well it is if you have it repaired by me :)
Dave Fennessy, April 2009