I have several Saito engines, including the .91 single cylinder and I have not had any issue with the muffler, even though I now that some people have had the same problem as you. The way I tighten the nuts is when the engine is hot, just be careful and use proper gloves for the task.
Recently I started to use Gasket sealer (the red one) on the threads of the pipe just to avoid the nasty burned castor oil get impregnate onto the outside of the muffler. Engines look better I do not know if this sealer helps to maintain the nuts tight, but as I said I have not had that issue so I could not tell the difference.