i would check ebay or watch the HQ for one. they go for next to nothing most of the time. pm NYUK, he might have one layin around.
im pretty sure the rz350 crank is the same and you wont have to change any gears, but itll prolly be easier to buy a used one for a banshee.
you can rebuild it but for themoney youll spend you can buy a used one for less im sure.
all you should need is a case for the bottom. if its not real bad you could prolly have a mchine shop weld it up and smooth it back out. i would really check into that before you buy anything.
ebay or here.
ebay for crank and rockymountainatv.com for pistons and whatever else you might need.