Selectrics are very complicated, as you might imagine. I don't know that I'd even try to fix one myself. Outside of looking for obvious mechanical problems. Like a corncob stuck behind the ball or something. That I could fix.
When I was a kid I was a printer for a big insurance company (are there any small insurance companies?) and there was a pile of Selectrics (a small pile, but still a pile) in one of the storerooms down in the basement, where I worked. They were very expensive typewriters in those days, the late 70s, so I asked my boss, "Why don't they fix these?" "It's less trouble to replace them," he said.
Which is my long-winded way of saying it's probably going to be really hard to fix it yourself. You may end up taking it to some weird old guy in a dark shop somewhere and begging for the voodoo. But if you do that, he's probably going to give you a list of a dozen other things that need to be fixed or serviced...
In other words, why bother?