I would think the .09 would be a bit under-powered, with the heavy radio equipment back then. Maybe a Tee Dee .09 (no throtte, just 2 channels...or perhaps an Enya .09, which are nice little engines )

Wing loading is probably the most important part with small planes like this....and after thinking about it some more, your O.S. FP .20 might actually allow it to balance better? (not only Center of Gravity wise, but over-all balance of power to weight, and general flyability)

If the Irvine .25 is a ball bearing engine, it might be a bit on the heavy side...with the muffler especially. Around here, we rarely used mufflers back then, on the smaller stuff.

What are the weights of the .20 FP vs. the Irvine .25, just out of curiosity...?

Have a look in the "Clubhouse" for the long thread on RCModeler Magazine...there are guys there that have virtually every issue, and might be able to put you in touch w/ a way to get the plans.