I hear you too. It would be much more convenient if there were some sure-fire method of determining quality levels before laying down our hard-earned cash. As things stand, I haven't found it yet either . . .
I completely restored my R90S almost seven years ago, and in the course of the restoration I bought stuff via the interweb from Australia, the UK, the USA, Germany and the Netherlands. Some stuff was OEM, while other stuff was aftermarket such as S/S mufflers, the seat, alloy indicator bodies, S/S indicator mounts, enamel badges, S/S wheel spokes etc. I found that everything fitted together and worked as advertised. By sticking to mainstream vendors and brand-names where possible, I believe that most of the risk was averted.
And, more importantly, I can say that I'm still very happy with the quality of the after-market stuff seven years later.
Incidentally, although I have already stated my preference for original, well-maintained machines, I will also say that there is immense satisfaction to be derived from taking a fully worn-out and completely dismantled basket case and returning it to the road. It is also very rewarding to clock up the miles on a "nut'n'bolt" restored bike. But for that bike to also then be impartially awarded at a classic bike show or featured in a bike magazine article can be only described as the cream on the cake.