Go with the Magnum .91. Mine flew with plenty of power with an Irvine .53, but the plane really came alive after I switched over to an ASP .75 two stroke. Wing loading is still lower than many of the so-called trainers.

The down side is that you will need to get more prop clearance for the larger engine. I converted mine to a taildragger so a 13x6 clears the ground with plenty to spare.

Yes, WM really should call the pair Super Stunt 60 and 120, instead of 40 and 60. I have both.