One thing i can tell you i have researched and looked in to the moki 30cc and i can tell you now there would be no slouch in power for your 60" Biplane. The moki engines are incredibly compact and very light.
And there not all that exspencive you can get one from germany without paying import tax (persuming you live in the uk)

WARNING WARNING!! DO NOT GO NEAR ZENOAH, THEY ARE FAR TO HEAVY AND THEY ONLY HAVE HALF THE POWER AS OTHER MODELS OF THE SAME SIZE.