The engines are all the same, no matter what claims of superiority are made. If a seller can't explain HOW OR WHY their kits are better, they are just blowing sunshine up where the sun don't shine.
Sometimes (usually) the cheaper engines have cheaper components and skip on things like left handlebar grips, different quality sprockets and exhausts, and chaingards.
Some like thatsdax F80 SUPER MOTORIZED BICYCLE ENGINE KIT MOTOR MOPED !! - eBay (item 230310705384 end time Dec-07-08 15:17:01 PST)
have much better components and seem to run smoother and have an overall better look and feel.
BTW- thatsdax is one of few (the only one?) who claims the 67cc engines are 70cc's, all others "pump it up" and say they are 80cc's. Don't be fooled by that, they are all the same.
Livefast motors is another good one.
Some have had decent dealings with boygofast on ebay too.