I've noticed that with the vintage Tecumsehs that the muffler/exhaust gasket is hard to find as well.
On most of them the muffler/exhaust gasket is actually the same as the intake gasket so you could just buy an intake gasket and it would work perfectly. It doesn't look like yours is that way though.
My only guess as to why it is hard to find is because Tecumseh thought the mufflers would almost never get bothered with since you rarely have to service them, so there wasn't a need to make an exhaust gasket since they wouldn't be able to sell for them to make a profit.
I'm no Tecumseh expert, this is just a reasonable guess. Hopefully someone on here might be able to give a great answer.