If you are looking for a chinese 2-stroke kit, zoombicycles is the best out of 5 engines I have had... and I got it on ebay. The grubee engines are just as good but the accessories suck.... especially the ones from bike-berry, for some reason I have had problems with two of their carbs so far. Sick bike parts has the good stuff you need to upgrade your bike... I would recommenced minimum; a new fuel filter, petcock, head studs/bolts, and an hd air cleaner. If you have a bike with gears or just want to run one chain to the back thier jacksfhaft kit is excellent.
If you are talking in general... I would look at the golden eagle bike kits. They are hands down the best ones you can get IMO.