i did it once--just to do it, I made notes and drawings when I unwound it, it is easier to find good used armatures, generator cores can be had for less than 10$, fields are easier, you do need a form, and a method of counting the turns, a sunnen hone can be adapted to make a winder by adding a counter.
From all the troubles I read on this site about alternators and the modifications needed to make them work reliably I would say the generator properly rebuilt is more reliable ---I am still using the one i rebuilt 40 years ago untouched ---and I have abused it at 15+ amps for many hours, same waterpump, same belt, Halogen headlights.
I do have some amp draw ,speed,type of data, but I don't think I have winding data, I will look.