Hi there!
First I'd get confirmation that you've got a crack and not just a casting mark/flaw. A leak from a more traditional place -- like a seal -- can migrate down a cosmetic flaw and make the flaw look structural.
Otherwise, it can be fairly technical getting the gears properly shimmed as you move them into the new punkin. If you're not completely comfortable working with shims and mesh patterns, there are folks that specialize in setting them up.
You might save some grief by buying a complete unit from one of the usual suspects out on the Ebay.