I think you can salvage what you have, if the rest of the engine is in decent condition.
If you use JB Weld to re-attach the broken piece (and seal where it broke the breather chamber), you can reinforce the carb mounting area by cutting a piece of 10-12ga steel to the shape of the intake gasket.
I would then epoxy the steel plate to the aluminum block (being sure a seal was made around the intake port) to eliminate the thickness of a gasket between the plate and block.
Use a single B&S intake gasket (some B&S engines had a double gasket) between the carb and the steel plate.
If the upper carb mounting threads are stripped in the block, they can be re-threaded for a 5/16"-18 bolt or filister head screw.