the case does need to be sealed well and standard contact seal bearings don't always cut it. only reliable way to use these is to leave both seals in place and rely on the grease for lube..... the back end (clutch side) can simply be sealed off with a plate or with whatever you cook up for a mount....but only if you cut the unused part of the crank off. front side can also have a plate made to hold a standard oil seal that will do the job just fine. pretty sure you're going to need something to retain the bearings anyways. and the case is most definitely magnesium. my favorite tool for doing the rough cutting is a good bosch jigsaw with bimetal aluminium cutting blades. a good band saw would probably be even better. save all the scraps for a bonfire...