Any engine that runs right. OS 61SF ringed is my favorite along with the 46SF, and the 61FS, 91FS and 120FS. Have a great running Webra 120 in a 80" Spitfire that calls for a 1.60 but it is plenty of engine for it. The OS 40 and 46 LAs are great trainer engines. Just got an OS 46AX on a Skyraider at auction that was really stiff but oiled it up and it runs great. Don't know that it has much over the OS 46 SFs
though. Have a ST 2000 in a Zero and it is fine also.