High temperature silicone, the red one sold in tubes (dental paste-like) (by Loctite, I believe).
Easy find at Wal-Mart.

Never a leak; I even use it for the joint engine-muffler with no gasket.
No need for strong forces, just clean any excess after applying it.
Easy disassembly as well.

Now, some stains will come from the return flow of the carburetor.
The air-fuel mix goes into the nozzle while the engine breathes, and then a little of it goes out, which ends up over the hot surface of the engine and muffler, creating stains.
Hot water with some dishwasher powder will clean your muffler (do not expose it for too long).

I believe the muffler leaks do not affect performance of the engine; and yes, you can use the muffler from the 0.46.

Regards!