That dang mac 555 would make your ears bleed coming out of that little square exhaust deal.I made up a replacement exhaust port gasket to fit in where the original had gone missing on the old ProMac 10-10s/ It's a fiddly bit, just a rectangle of material only 1 3/8" on the long side, and a large rectangular hole on the inside. A little picture frame, not even any bolt holes the gasket fits in a recess between some raised ribs cast into the cylinder boss. I should have captured a picture, but...that's not me.
I used soft copper sheet, cut out with a sharp chisel, using the vise jaws as a guide. A fine rat tail file provided clean up. I didn't take the air shrouds off, So the fit was observed through a couple of fastener access holes in the sheet metal shrouds. I think I got pretty close to good.!
Tomorrow I'll gas up and see if it's a go. It usually just takes a squirt of fuel in the carb to get it to pop!
Really only saw I can think of that hurt my ears before.