It is indeed very time-consuming and one needs to have a really first class ear, but it can be done. Perhaps not wholly accurately, but pretty closely. There are a number of works I personally would like to see reconstructed in that way - the PC by Lesley Bridgewater for one. However, it would cost a lot of money because fellas who've got what it takes will not charge peanuts (and why should they?) unless it's a work that is particularly dear to them.