dont buy cheap ebay kits.
my only ebay kit, $129 for a 50cc kit.
it arrives, cool! its a 66!
install. make pipe. and guess what? it siezed. it died, in a week of use, not once did it make it the 5km to work. orderd some parts, to find that everything was different again to the (two) common styles of 66cc engine. one day its running stinking rich and fuels dripping out the pipe. the next it was running so lean the plug electrodes melted, yet it was rich when i got home andd started it (the carb was fine, i even still use that carb on another engine... gaskets, etc, all the same...)
just a lemon from the word go.
meh. anyways. while they seem to be from one or two manufacturers only, im sure theres a lot of middleman that have no issue with buying and selling the "scrap" engines, that dont meet QC (skyhawk i believe has some level of QC) and im sure the manufacturer doesnt mind, as they are making money from what would have been a loss.
did i mention my "cheap" 66cc happened to be made from an alloy that felt softer than the base of a tealight candle? i could bend fins by pressing on them. it sounded like hitting a lump of lead. not "ting" like good alloy casting... but "thonk" like hitting a lump of modelling clay..
and yet ive had genuine 50cc (48cc) engines that have been literally run into the ground, and still fire up, still run, even after 10,000km+ on them, at 50:1 oil ratio...go figure.