Have you ever tried to generate parts from a physical score by photocopying the score, cutting each part into strips, and then pasting them together on a sheet of paper? I recently discovered Parfiti, a free online resource that will essentially do the same thing electronically. Using Parfiti you can take a PDF score you have on your computer or a file from IMSLP and use the score to print out separate parts for musicians.
Using one of my own scores converted to PDF didn’t work so well. Parfiti doesn’t handle landscape orientation well as it currently assumes that the score will be in a portrait orientation. But going through a portrait orientated score worked great. You can even just copy and paste in the IMSLP ID number and Partifit will import that score automatically for you. You’ll next need to label each of the parts, but once you’ve got them properly listed it will separate the parts for you for easy printing.
If I needed to replace a part that was lost I’d probably find it faster to simply create that part using Finale, but if I needed to generate many lost parts from a score I think Parfiti would be a faster tool, particularly if you don’t have access to notation software or just aren’t all that quick at using it.