I’m a total newbie when it comes to small engines.

I have a motorized bicycle that I’m building for my father out of his 1960’s schwinn hornet that he has had forever. I have the 50cc 2 cycle engine and it has a rectangular exhaust port that is 40 mm center to center on the mounting bolts. The port measures approximately 23mm x 15 mm. I tried a generic compact muffler that was made for a brush cutter but it doesn’t allow the engine to breath. I’m looking for a more open exhaust.

Could I use an exhaust with a circular flange to mount up to the square exhaust manifold?

Is there a such thing for a circular to square manifold adapter 🤷‍♂️🤔🤷‍♂️?

I ask this because there are a plethora of muffler/exhausts that have a circular flange that bolts up to the engine but doesn’t match the rectangular manifold on the engine.

Any suggestions/ recommendations would be greatly appreciated!