I've got one of these, had it for years....like when they first came out....ran a OS 61 4 stroke on it until it wore out.....can not say how many gallons of fuel was ran through it....but alot....just recovered it this past year....and installed a OS 70 Surpass II on the front with a 14 x 4w apc prop on the front..will pull it straight up as high as I want it to....in my mind a 91 would be just a little over kill...after all this isn't a 3D plane...and from having the 4-40 which I have also with a OS 52...you know that by setting the cg back it'll do about anything my fingers can keep up with...I did build another wing for the 4-60 and did exactly what you did or are going to do...I've got just shy of 1"...was going to shorten the wing by one rib bay, but didn't...had the Ace 60 Bingo, which has the same wing, and shortened it one bay and it was alot more responsive...I also installed two aileron servo's in lieu of the one shown on the drawings...If I knew how to post pictures here I'd would...if you wish send me your email address and I'll send you some...