Permatex High Temp silicon gasket maker is what I use.

It's not a muffler cover - it's the transfer pipe cover. It's a U-shaped exhaust pipe that carries the through-the-deck exhaust back up into the muffler inlet. There is no particular urgency to having it sealed at all. At worst you would get a minor exhaust leak under the mower if the transfer pipe was leaking. If the exhaust ports are clogged any at all, be sure to also check the transfer pipe and muffler. Clogged ports on this engine are rare, but possible depending on if it's been operated in a rich condition for extended periods.

I would suggest trying out a Dow Corning silicone sealant for this task. You can check them out to see if they offer anything that could be helpful to you:

Here is a link that might be useful: Silicone Sealant