Hi Linnix, these kits are made in China, the reason the kits are made is simply put, is to make money. The kit people seem to not care about the end results, or the quality replacement that you desire would have been made. There are apparently none of these being used in China, so they do not suffer for the lack of quality. There is a saying that is "TIC" meaning "This Is China", and the secondary meaning is "things are different here".
I don't seem to know who "they" that you were hoping would rise up, and make the tranny that you wanted to buy cheaply, but options DO exist.
I pointed out to you that Firebelly has made American boxes (tho I do not know if there are any to be had at this moment).
You can also check with Dave Staton, and see if he has anything for you, but, his gear-reduction is USA CNC and will not be at China-prices.
I believe that I told you the impetus to start the EZMotorbike Company, was to give American owners, (and whomever we can sucessfully ship to), a quality primary drive to use behind what is a very good little 49cc 4-stroke engine, that has the capacity to be an outstanding transportation machine. We started with the Silent Power manual clutch, and now have a new auto-clutch model, the Q-Matic.
We had hoped to be able to manufacture for less money than it turns out that we are able to. The current production is CNC Aluminum transmission body, American clutch (custom tuned), and all other parts sourced from USA vendors. This all really does cost money.
These as best as I know, are the options available to you, unless you can design and build your own, or, maybe, you could use a scooter or pocket bike unit,I do not think it would be easy tho I have no first hand knowledge of what you would have to do to modify one of those.
Good luck, as far as I can tell, this is where we are (I ride both Silent Power, and Q-matics myself).
Mike