You can't go wrong with either the OS 46FX, or the 50SX. Also the Supertigre engines either the 40 Ringed, the 45 ABC, or the 51 Ringed engines will give years of good service if you run them on the rich side, and they are very inexpensive. Stay away from the Tower engines, they are chinese made knockoff of OS, very inexpensive but the tolerances are way off

There are tons of other very exotic engines in the 40 size category, but for most sport flying these engines are more than adequate.