re: which is the best kit?
I have burned 3 engines already. And I'm still waiting to burn my engine with Rock Solid HP engine. It has stalled due to overheating, but after cooling down. It started right away and still kept on going and using it to this day. I use my motorized bicycle almost everyday, and have the engine from Rock Solid for more than year now without any major breakdowns. The drive sprocket is unlike any other kit I have. It can't be filed down to have a smoother chain action. You'll have to use a grinder and it does take some time.
Regardless if it's left or wrong, the opposite is always right.
Caution: This hobby is habit forming.