Hi Joel,
Fixed gear is the way to go, in my opinion; a lot less maintenance! NovaRossi engines have been getting some very positive reviews: http://www.klasskotemo.com/NovaRossi%20Engines.htm
You can also call Rich Verano at YS engines and ask about the YS 61 classic. The engines were sold out, but Rich was planning on building some more from the stock of spare parts. I have been running the K&B Aero 100 which is a rear exhaust engine. It has the same mounting bolt pattern as the OS 61s, but the case is a tiny bit wider. It is a real power house with a pipe and headers and pipes are available from Macs Products. I tried one last fall and put about 100 flights on it. I ended up running an APC 13.5 x 10 and it turned that prop with real authority (I don't use a tach any more, but I would estimate the rpm at over 10,000); the plane it was in was a tower hobbies Apollo 90 and it had unlimited vertical. Good luck with your build! Post some photos if you can.