Thanks for the tip on the Duratrax.

Thanks for the other tips, but I've already done my makeshift repair.

After seeing how much a new muffler and header cost (~$56), I decided to fiddle with my old one. I degreased the header and the nonexistent threads on the muffler. I had already drilled the muffler and header, and I used the holes for a 4-40 screw and double nut + Loctite. Then I tightened the header's retainer screw (+ Loctite) down against the muffler. No vibration now! And finally, I used a 19 g retention wire.

On the engine side, I degreased and used high temp Loctite.

When it falls off next time, I get a new muffler/header and follow your suggestions.

The next 4-str engine I get will have the pump.