I had a .55AX in my Decathlon. I would say that the engine you have is at the low end of the appropriate engine, and the 55AX is at the high end. Both will work, as will anything in the middle. After having flown a few full scale planes, I now better understand what these planes are meant to do - and a .46 will do that just fine. A .55 will help you gain altitude a little quicker. I have had good experiences with OS engines and OS service. I like the .55AX a lot, but personally I would go for a four stroke in that airplane if I were to do it again just because I like four strokes and the sound would be a little more scale.

I suggest not buying a new radio until you really understand what features you want, besides 2.4ghz. A good radio should last you a very, very long time. Buying something just to get 2.4ghz then having to replace it because you want additional features is a real pain. I wish I had waited before buying upgraded radios as I'm now shopping for my 3rd radio in 2 summers. I STRONGLY suggest downloading manuals to the radios you are interested in, and read them carefully. Figure out what you want the radio to do - EVERYTHING you want it to do, then read how to do it. See which ones seem more intuitive, have the right features, etc. Go in the forums and find out what people DON'T like about the radios. Take your time with buying a radio. It should not be an impulse buy.

I will be selling my Airtronics RDS-8000 with 3 receivers and upgraded battery in a few weeks. There are lots of good 2.4ghz radios available at good prices, new, used, and discontinued.