Has anyone had experience with the RCG 15 gasoline engine? I am thinking of using one to replace a .90 four stroke; similar power? An it looks like it comes without a muffler; any ideas on a supplier for one?

You have options. Take a look at Evolution and O.S. for their 15cc engines.

I've got 2 of them and haven't had any issues with either one and they do come with a muffler.
My only advice is to be aware that the Tillotson carbs that are on these engines seem to require
a somewhat richer setting on the needles than do a Walbro.

I have one. 14*7 prop is about right. Very reliable, good throttling and almost similar power to an ST90 glow. Trouble free.

DLE20 is LOT more powerful for similar weight, but costs more.