KISS.
SIG's Akromaster built per plan and built light powered by a lightweight moderately powerful glow engine (light case cross flow PB = perfect) is as per Mike Gretz's design intention and will fly like a miniature Twister at a comfortable pace on 52' lines. This should be the objective and ideal. If not, you've chosen the wrong design for purpose.
Schnuerled .10FP-S or a Schnuerled .12 TBR intended for R/C isn't a good sub for a PB as described. Power isn't all about claimed peak PS.
For an .09 or .10, there are simply better performers to build which will perform better on their power characteristics and output than an Akromaster, and for most of the alternative .15s, same applies. Smousen and Peacemaker to name but two venerable .15 designs are far more accomodating and flexible for similar purpose to the Akromaster. That said, any of an 15LA-S, 15FP-S or TT GP-15 will fly an Akro well enough even with muffler aboard on an 8x4 as long as you keep the combo light. But they aren't made and sold NIB any more either. Brodak's Schnuerled .15 which is available seems a reasonable sub for the latter three. Will other engines get it off the ground and rotate it around the circle? Of course they will, but not to my required aerobatic capability and handling standard on 52' wire. YMMV.