ORIGINAL: Sport_Pilot
Any more than a good .40 is too much. I had a TT .46 on mine and it needed way too much down trim unless the engine had a good stack of washers pointing the engine down. Switched to and old K&B .40 and it was just fine. I would suggest a TT or ST .40.
Or if you prefer a four stroke, then a .52 to .60 four stroke.
I had one with an old K&B 40 tooBuilt it from the kit years ago. Flew great and was very rugged.(I tested the durability several times.)
I gather the ARFs are heavier and less durable though.
jess