Redoing my roof: how should I redo my insulation?
Unless you need to, don't rip off the sheathing. At 60 years old, it may need it anyways. If you look at the roof can you see dips between the rafters? If the sheathing is still flat, keep it in place.

Your best bet is probably blown-in cellulose. You can hire a company to handle it, or rent the machine from your home center. You should only need a small access point to get the nozzle in, large enough for a rake to smooth out any irregularities.