The o.S. 25 FP is a great match for the Sterling Ringmaster, Yak-9 and P-51. Smoother running than a Fox .35, and starts easily and consistently.
I have used this engine with good results on several planes originally designed for the Fox / McCoy .35, and all flew well. It is also a great match for the ARF Flite Streak.

I use a tongue muffler to save weight, but the stock one works fine.
Hayes 2-3 oz plastic tank set up for clunk feed and muffler pressure, 3rd line for fill vent that is capped for flight. Fill it through the feed line and you won't flood the engine / fill the muffler with fuel.
.015 x58' lines and a 9x4 APC prop are good starting points.