Hi Mark,

 An IO470 would work in a FG Legacy. Some of the early IO-520 had the same crankshaft and crankcase casting and a IO520 I am told. I have a friend of mine who builds airboats and he bores the cyl. bores out larger and bolts on 520 jugs. He also says if you deck the case where the cyl. bolt on you can get the compression up to 10/1+. He estimates around 330 hp with a stage two cam. He buys core engines for around $1500. He tells me his engines stay together. NOW, I am not advising you to do this exactly because we are building aircraft not airboat engines, but I just wanted to advise you what is working. Also P-Ponk has an FAA approved STC to convert 470's to 520's. And the 470's are a little lighter weight engine.

 Also there was a Panzel aerobatic aircraft with a 470.

I think it could be worth more research from some professional aircraft engine builders like Barrett or Lycon.

You get a lot of different information from people who don't have first hand knowledge here on the list, no offense.

Swaid