Thanks for all your suggestions! A thought I did have was to use some loktabs - like motorcycles and machinery with vibration problems but there is no room in reality on this engine to have the fold-up type. I considered safety wire like they use on aero/jet engines where the wire passes through the nut/bolt head then pulled tight into the next nut/bolt - like one of you has already suggested.

I think the best option here is to make up new studs, weld some standard nuts to the top with flat washer underneath and drilled to accommodate safety wire.

It's getting a right pain in the proverbial having to keep removing the prop and cowl to tighten the muffler.

Thanks for your help - Especially to avoid nyloks!