They're a lot better than when this post first landed, but still not to Japanese build quality. That said most of the big 4 have satellite factories churning out their small capacity bikes in countries like Thailand, India and Indonesia. Or they badge engineer Chinese built bikes. As the build quality of Chinese bikes has steadily improved, the quality of Japanese manufacturers has steadily declined, to the point where it now depends more on how well you look after it as to how long it will last. Brands like Sym and Sinnis are turning out some fairly decent bikes.
50cc 4 stroke putting out 3.2kw / 4.3hp, that weighs 121kg, doing 45mph is possible but unlikely mate. Derestricting a 4 stroke engine with such little power in the first place won't give you any significant gain: not like a 2 stroke. It's fairly straightforward to remove the bridle / restrictor if you do a search for it, but there is no credible evidence of the advantages gained, other than anecdotal claims of 70kmh (42.5mph ish).
It might be an indicated 45mph downhill, but it's unlikely to be real speed, and certainly not with two of you on it. Added to which you will significantly shorten the life of that little engine.
Oh and if you got caught you'd be in a fair bit of bother. If you're riding a 50cc made under 3dld rules (01/14 i think) it has to top out at 28mph.