That kit seems expensive, maybe im wrong but im sure a 72cc kit is less than that so wouldve expected the stock 50cc to be cheaper?
The seller is good tho, i can vouch for them. I got a load jets, plugs and other bits & bobs off them and was always a quick delivery and good parts.
Maybe try CMPO website (chinese motorcycle parts online) they may be cheaper.
Think the guy who sold you it mustve known it was on its way out and passed his problem onto you? Or maybe it was just co-incidence.
I bought mine when it had around 7,000km and, touch wood, its been great so far, up to about 22,000km now and feels better than wheni got it.