OK, so run the engine without a silencer.

You should be able to fabricate a short stub-pipe, which attaches to the head ... it may be that you can cut a completely different thread to that which exists; a Metric Coarse thread might do the trick. Once you have a short stub, permanently-fitted, you can lead away the exhaust goo.

It all depends upon what facilities you have.

I run several engines on straight pipes and I find the sound quite acceptable.