Dave, I have a Moki 1.8 in my Yellow Spit. I don't have a lot of time on it (even thought its 3 years old) but when it flew the last time it flew pretty well. It's not over powered with the 1.8 but flies scale. I am using an 18x8 prop. A friend is flying the same plane with a Mustang 50 cc gas and a Bolley prop. Lots more speed and vertical performance with that set up.
I have a Slim Line pitts style muffler with My set up. I had to rotate the engine about 10 Degrees and releave the fire wall about 1/8 inch so the muffler clears it. I put a duct in the Fuse of the plane with an exit at the oil cooler on the wing bottom. Air passes in thru the cowl, across the cylinder, thru the duct via an opening in the firewall and out the bottom of the wing. I am also using an onboard igintion for the Moki to keep the plug hot when at low idle. FYI I am currently building the Yellow Bubble P47 from the 78 FG also. I am doing Frank Oiler's "Eileen" form the 84 FS. I did considerable reasearch on the paint of that FG and the defenitative word is that they were RAF Dark Green uppers and sky lowers. This was confirmed by Gary Fry who is the group historian and authored the book "Eagles of Duxford". Thanks for the reply!!! Regards BobH.