My suggestion is to get a Hattori that is designed for a 50 and try that.

I have a Hattori and it is great!

If you still have the original Hirobo muffler you can get it to work. First disassemble it. Clean the bolt and back side.

Mix up some JB Weld and use it to seal the 2 halves. Reassemble using RED locktight on the bolt that goes through the center. Lastly use a self locking nut on the last bolt end. Has worked great for all I know that have used it.

Clutch shaft runout is normally .002 MAX try to get it less if you can. I had to open up the holes a tad to get it so I could narrow mine down.