Cases that are corroded at the sump flange are usually rotted from water that sat in the motor a long time. Probably from sitting out side in the rain with the air cleaner gone or oil cap missing. If you have enough flange to seal the gaskets and screen it should be fine. But you need to determine if the case is ok to use . I would check the center mains with a straight edge to see if they are sunk. put a straight edge on the front main and the center main and see if it runs into the rear main instead of running across the top of it. also squeeze the head studs and check for cracks on the flywheel side of the case and also in the cylinder bores. If the case is clean and there is a crack then oil will bleed out of the cracks. Is the case line bored a bunch. If it is chances are it has a lot of miles on it. check the lifter bores for excessive wear.
Check the long 8mm case studs to see if they are stripped out or ready to strip. Oil pump mounting surface? The Robert Bently VW manuel has all the
tolerances. good luck on your motor,