One other 26cc engine that's very strong although somewhat expensive is the EVO/MVVS 26 gas.  I have both the Zenoah and the MVVS and the later turns an 18X6 XOAR 3 to 400 rpm's more on average than the Zenoah.  (Zenoah 8500 to 8600 and MVVS 8800 to 8900) They both run fantastic and the Zenoah makes just over 18 lbs. of thrust as measured with a fish scale with the engine on my Spacewalker.  (The engine and plane are seen in my avatar.)  The MVVS is in my Mayhem weighing 10lbs 4oz and hovers at just slightly under half throttle. (It's hard to say how much less than half since my thumb is always moving the throttle while hovering.)  Excellent punch out from hover and I get 20 min flights on 10oz of fuel with a little left over for good measure.  The MVVS/EVO is also lighter than most other 26 size gassers.  IMHO it's biggest drawback is price.