Sporty-

I built the Citabria back in 1976 and competed with it in the Stand Off Scale event at the 1976 Nats (at Wright Patterson AFB).
At the time I used a Veco 50 (no longer made) which didn't have too much power as it was an old engine, but it flew it well.
Couple of years later I changed it out to a OS40FSR - more than enough power. You won't go wrong with a 40.

After competing with it a number of years (and doing well) I put aside and didn't fly it. Last year I donated it to the local airport where it now hangs from the operations office ceiling .

Good luck!

Maine Bear