Put a thick spacer inside barrel exhaust before pipe....
You can even make a new spacer like pic.
...
Make either, but make inside diameter smaller than stock. It cheap to try different diameter, but start although 2-3mm smaller.
Honda use on 500cc works bikes exhaust flange with different inside diameter depend of track.
I have my own experience of that too. I ride sometime earlier DirtTrack (1km=0,62mile track) with my then bike (
Honda CR480). It has bigger hole in exhaust pipe than exhaust duct in cylinder. Of course
I make to exhaust duct same diameter as pipe has. Everything else work fine, but starting out gone very very difficult. I put 2mm? thick washer with original exhaust duct inside diameter between pipe and cylinder and starting out work again fine! I was very happy then.