In my experience (while somewhat limited) I've actually found the contrary to be true when people says "you get what you pay for". I guarantee most companies you can buy a 49 or 66cc engine kit from for more than double the price as some people have suggested you do, will be absolutely 100% the exact same thing, probably purchased from the same overseas vendor, and just given some stupid name, maybe a $0.5 sticker you'll likely remove anyway, maybe specs inflated a bit, possibly if you're lucky $8 in new bolts and a 200% price increase. Don't waste the money on what will regardless be an inexpensive engine designed to be an inexpensive engine no matter what some greedy schmuck prices it at. Sure, there are some companies that do actually sell "high performance" engines(still a lot of times with extremely inflated horse power ratings and such or basically just gestimimates from never having actually put one on a dyno) but again, not really worth the money in my opinion when you can buy a larger engine that has a higher power output than a 66cc engine ever can for the same price. Then again I also see no logic behind spending $1000+ on a bicycle with an engine when you can buy a used motorcycle for that, so maybe I'm just biased in my thinking to keeping what is designed to be a cheap mode of transportation cheap.
In any case I bought a $69 kit from eBay (sellers name was radiatorpartscenter, it looks like their price has gone up since though) and it's been great so far and is an exact 1:1 match to the pk80 my friend spend $142 on from one of the well known US based vendors during a "sale". Surprisingly the eBay seller was far more helpful and accommodating than the large company charging double, and even split the cost of a new $22 chain with me because the one that came with the kit wouldn't work with my frame, which I really didn't expect. Also shipping was 2 day fedex to my door for free.
So in short there is definitely no rule, especially in this industry, that says you get what you pay for.
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