Open them up and take the guts out of the new one and put them in the old one. Bearings are probably fine - just put a few drops of oil on the felts. No need to remove the windings unless you know for sure there's a break in them. Usually the only things that go bad are 1) the brushes wear out and 2) the commutator gets worn and scored but even that can be cleaned up in an electric drill with a file and some emery cloth.