1984 ytm 225 exhaust and other questions

[Mach-10]

My 225 starts easily and idles fine. Randomly it misses and there sounds like a knock coming from somewhere. At most it happens once every 30 seconds. Any ideas on what is causing this ? I was told a blown head gasket can cause it but when I have the three wheeler running I feel no air coming from around the head gasket or where the exhaust goes into the head. I do see that there is no gasket where the muffler and pipe join and it is leaking exhaust here. Anyone ever seen this cause or knows what is causing the issue I am having ? Or anyone know of a way I can fix the missing gasket without paying $30.00 for a gasket?

[Mach-10]

I covered where the exhaust pipe and muffler connect with aluminum foil to test it out and that seemed to fix the issue. Anyone know where I can get a muffler gasket? The ones in eBay are 30.00 and usually when I get something off eBay it's the wrong part. Thanks.

[redsox]

that was the cause of the knock?

[Mach-10]

Yep. It wasn't a constant knock. The three wheeler would miss every so often and when it did you could here a single knock sound. I covered the exhaust leaking there and it stopped.