With regards to the muffler.
As they are a bit rare, and untill you can get hold of a good one.
I would make a patch, some steel in he right shape, and even put some head gasket off cuts between and screw it all together..... it'l work.
Here's one I did on a Victa..... 125 Mayfair.
[Linked Image]
Little self tappers .

I had one of those in the 70's , smooth running and reliable .........

good luck, it will go
speedy

........................Keep your blades sharp......................