the ringer circuit in most phones are relativity simple, a 470nF capacitor and a 1kohm or higher ringer load directly on the phone line will work
depending on what went wrong you'll need to measure the bell coil(s), it should be about 1k total and 500ohms per coil, if that measures fine test the 470nF cap, if its open or measures low capacitance you can replace that also make sure the hammer moves freely and strikes both bells
if one of the coils are open, depending on where it went open you can repair that just peel back the cover and find the winding wire where it broke and extend it or unwind it a few times and put it back on the solder point