If you build light a Ringmaster will fly with a modern .15. Most (I think) use a .25 which has all the extra power you need.

A modern .40 is great, but not for such a small plane.

As someone mentioned, a modern .25 also vibrates much less than the old Fox .35. If you are building it for Old Time Stunt, the Fox is great. You didn't say which model you have, the S1 is eligible for OTS, the S1-A (the one with the sheeted leading edge) is not.

I have a Fox .35 on my S1.

George