hey oldtimer,

if you are talking about the ARF Lancair, then you really will want to go to the larger engine.
the .55ax, albiet a strong engine, is probably going to need a half pound of lead to balance, and will at best, just get the plane in the air. now there was a thread in the pattern plane forum where a builder used a .55 in a Dirty Birdy which is essentially the same size, but lighter in weight (by about 2.5 lbs.), and it flew fine, but it also has a lot less drag.

the plane calls for a .61-.75..............the .55ax will swing a large prop, and that is what the AX series is aimed at (bigger prop, lower noise), but still will be on the weak side of a .61.
i fly 2 of GP airplanes right now after just getting back into the hobby after a 15 year layoff (benn flying since the late 60's), and i use the upper end of the recommended engine range in those birds.......niether (Big Stik, Super Skybolt) are overpowered at all............both could use more.
if you decide to buy another engine for it, i'd go with the .75.ax..........very impressive engine and will swing bigger props than a .61FX.

also, "tooling around" as you say is much more enjoyable when you can do it at less than full throttle.................odds are very good that you'll be hanging on the prop most of the time with the .55.