Re: Help.. Broken Bottom engine casing.
The broken, missing part is not in a critical area. The opening will allow dirt and grime to enter the clutch areas but it could be sealed with something. There is no compression loss or air leak into the intake at that point. Before you go to all the work of rebuilding the engine I would try sealing the broken area and riding the bike. If by some chance you have the piece that came out, it could be set back in place with a metalized epoxy such as Devcon Liquid Steel/Aluminum. That sure seems a lot simpler that tearing the engine completely apart especially if you have limited experience.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"