Replacement bolts are available. Its a metric thread bolt with a long unthreaded shank, and about the last 3/4 inch threaded.

If it were an O.S... the bolt comes with a replacement nut and lock washer. Should be the same bolt for the TT engine, if you can't get the TT part...

don't tighten it so tight. Use some "Lock-Tite" on the threads. The only way I know to break that bolt is to overtighten it. (But its easy to loose it if you don't use some form of thread lock compound...)

You can get a long 4-40 bolt or threaded rod, drill out the holes in the ends of the muffler for that to fit and use aircraft type nylon inserted locknuts... I'd avoid this unless you absolutely have to get flying before you can get the original type bolt.

There's other ways to jurry rig it and make it work... but best to buy the correct bolt.