Bike good:Muffler bad

the hole might have stripped out. it's supposed to have a welded tab inside, covering a pipe. hopefully that tab hasn't broken off as well. if it's still there, check to see if the threads in that hole are gone.

if they are, then you can buy a tap from Ace Hardware or anywhere that sells tools. get one the next size up, i think 1/4-20 will work on those. also, buy a 1/4-20 bolt, lockwasher, and flat washer (i'd bring the pipe with you, and match it up, incase the hole's bigger.)

on mine, a 1/4-20 tap worked, and i didn't have to drill the hole any bigger.

tap the new hole, put the cap on, and tighten that sucker up.

While what bairdco said is the best way - I found that unfortunately it doesn't always stay fixed. Even with a lockwasher and a retapped hole mine would rather have been bouncing down the road than stay put.

My short-term solution was a coupla tack welds to keep the cap in place (tack welds so I could grind them off should I need clean inside) - my long-term solution was to replace the cheap, restrictive POS with an expansion chamber and silencer.

I'd defo recommend the latter should it be in the budget, stock can falling apart aside - the performance gain of a proper exhaust system is worth it in itself

Get a new muffler.