If you don't need to restrict yourself to a .61 capacity, install either a .75 or a .91 with the same crankcase dimensions as the .61. Examples of engines worth considering are the OS FX .91, the Tower .75, the GMS .76 ringed, the ST .90 or the .91 Magnum/ASP clones.

Key to a successful inverted installtion is correct tank placement and a good carburetor with a dependable finely tunable idle. I can't speak for the Evolution's carb, but everyone except the most profoundly prejudiced would agree that OS carbs set the benchmark for tunability and consistency.

Of course, if you are restricted to .61, disregard the above!

FWIW, if I were putting together a pattern ship with a .61, of the two engines you suggest, I wouldn't hesitate to choose the OS .61 FX, because I know they carburate well.