RE: which is best 26-35cc engine?
I would go with a OS 33cc gas engine first, then a Evolution 33cc gas engine, and a Zenoah 26cc as being the best engines to use. The OS and Evo engines being more powerful than the Zenoah by quite a bit. The Zenoah is quite good but getting a bit dated at present.
The MVVS 26cc, or 30cc (and Evolution branded) are also really good . The Moki 1.80 and 2.10 gas engines as well. The RotoMotors are another one in this category too.
Then there is the other engines like the DLE 30cc engines of course. These are in the next level down from the top.
Engines like the SYSSA 30cc used to be the top tier engine, but they seem to be having issues nowadays, not with the engine per se, but with support. Mintor may be OK now, as they seemed to have gotten their act back together.