i've only run the "80" cc kits, and i've got some pretty amazing performance gains out of them. i'd like to do a 50cc motor, just to see how much i can tweak it. if i ever make it to bonneville, i'd like to run in the 50cc class, as anything above that would be impossible to set any kind of record with an HT, compared to what everyone else is running (unless they had a seperate motorized bicycle class, which they don't...)
from experience, i know the cheap china kit can survive a lot more than most people think, as the ones i've heavily modified are all still running strong (the newest one's only got a few months on it, so only time will tell how that monster holds up.)
i'd love to see an american made motor based on the HT, or even a motor based on older technology, with pushrods and all the cool stuff antique motorcycles have, but i don't think it's gonna happen due to the cost, at least not on a large production level.
i think the problem lies with the economy, and the american public's spending habits in general. seems like the majority of people have the attitude of "why pay more" when it comes to these bikes. as the most common bike to motorize costs less than the entire motor kit, those same people can't justify in their minds (or wallets) paying 3 times more for an engine.
on the same token, those same people argue, "why should i pay more for a quality (or custom) bicycle when it's just gonna have a cheap engine kit on it?"
i think that attitude is the biggest setback for the entire motorbike "industry."
the few people making custom parts are often knocked down by people who think because the motor kit is so inexpensive, every other part should be, too. gas tanks and expansion chambers are a perfect example. just because it's being made for a bicycle, doesn't mean it's any less work. look at prices for custom motorcycle tanks and aftermarket dirtbike (or moped) exhausts. the same process goes into building them, and the price reflects that.
another thing to consider with an american built engine, are the parts that go with it. one of the reasons the chinese kits are so cheap, is because of the quality (or lack thereof) of the components. the generic throttles, grips, levers, chains, gaskets, etc, are the cheapest quality available, mass produced and poorly designed.
if all those things were upgraded in an american kit, it would be triple the cost of the motor alone. i have a throttle on my bike that sells for $100+. i lucked out and scored it for much, much less, but it's an example of what things actually cost.
obviously, i've put a lot of thought into this, and i believe the motor isn't where the problems lie, it's in the quality of everything else. on my current bike, the only thing left of the kit is most of the engine and some of the exhaust pipe.
at this stage of the motor bike "fad," i think we need more quality aftermarket parts, which are slowly coming along, as well as a change in attitude as to what we're hoping to accomplish as these bikes gain popularity.
my honest opinion/prediction, is if the current attitude of these bikes being "cheap toys" prevails, motorized bicycles are doomed to be the next go-ped or pocket bike. i see more and more cheaply made bikes on craigslist as people grow tired of them, and then selling them for a loss. and then the next ad is a fly-by-night company with cheap bikes trying to make a buck off them.
sorry, didn't mean to turn this into a soapbox, but that's the way i feel about it...