it depends on how much $$ you want to spend.
you can buy a more expensive kit from an engine kit/bike vendor or a generic e-bay kit.
the prices are drastically different.
in my opinion all of these engines are the same and it's the luck of the draw if you get a good one or a bad one.
I have bought 4 e-bay vendor engines and all 4 of them have performed excellent with no problems.
one thing to remember is that you will give up customer service by going with a cheaper e-bay vendor engine. But if you're mechanical and you don;t care about customer service because you can fix the stuff yourself (like me) then customer service won;t be an issue.
another thing to remember is that ALL 2 stroke bicycle engine kits need to be tweaked to get them to work how they should.
this includes carb tuning, replacing the supplied nuts, bolts and washers with higher quality ones, using blue loc tite on everything, replacing the stock spark plug and wire, getting or making a better chain tensioner, soldering the ignition wires together and heat shrinking them. there may also be other things that will need to be made or modified to get the best results.
and then you can get into expansion chambers, race carbs, high flow air filters, milling the head for more compression....lol.
a big part of having one of these kits work correctly and last a long time is correct assembly, good wiring and lots of maintanace / tinkering.