"half breed" engines are the best balanced of the common 66cc types, but need an extra base gasket or two and the top of the cylinder decked by quite a bit to make any real power. PK80 motors make the most power from the factory, but often need considerable crank balancing. GT5 motors fit somewhere in between. any other designation like "flying horse" or similar are just brand names and typically don't mean anything. anything branded by grubee, dax, screaming roo, arrow, jake, or other "boutique" brands are the fastest and most reliable, but they all come with a price tag that's at least double what an eBay engine costs, which is already double what they go for from the factory.
the best reasonably priced engine now that rose is selling mostly lemons are the half breed motors from pistonbikes. if you want a real performer for under $300 without doing any really hard work get a half breed bottom from pistonbikes for $100 and a cr machine stage 1 top end kit for $180. then pick up one of mzmiami's snake pipe copies for $55 and you'll really be screaming