hi,
the zonger 140 is a better engine compared to the lifan 140 (my opinion)
lifan 140's have a tendency to break kick starter gears and occasionally snap crankshafts.
as for upgrades the single best upgrade would be replacing the carby with a 26mm oko setup
DHZ Mini Moto - OKO 26mm FlateSlide Race Carby Kit
as for other upgrades it really depends what is fitted to the bike already, like in the ways of ignition and exhaust...
theres really no limit to upgrades if you start upgrading the internals of the engine..
cam, hi-comp piston, big bores, lighten oil slinger and so on...
im guessing you havent found a bike to buy yet?
let us know what your considering and we can try and point you in the right direct..
after all there are alot of rubbish pit bikes on the market these days and you ALWAYS get what ya pay for..