Sounds like you've got a few top gear moments coming up here.... "ambitious, but rubbish"
Engine casings, covers etc. are all pressure die cast, which you can't do at home, and are probably too thin-walled to ever work as a sand-cast so dont even bother. If the sealing surfaces are buggered, fill the area with aluminium devcon. It sets hard enough to file\mill off flat. I think it will drill and tap if youre carefull enough.
As for pistons, dont go getting all burt munroe on us, cause you need to be able to machine them afterwards, which you also cannot do in your backyard. (don't forget, theyre tapered AND oviod so good luck with that one)
But dont be discouraged, it's worth having a go if you have nothing better to do, at least you'll learn alot.
If you ever get something worth machining up, drop me a line in Adelaide I'll be in it. Sounds like a laff