We would use the components from a 500 or 2500 set and neatly install them into the area of the phone that would hold the dry cell batteries. Use a magnetic cabinet latch to keep the door closed, but swing it open to reveal the dial or touch tone pad. You would remove the transmitter guts (save them for posterity), and put in a standard T1 transmitter capsule. The receiver element could sometimes be reused, but would sound tinny, so a heavy plastic earpiece with a U1 receiver element works well (again, hold on to the old earpiece). Old ringers are hit and miss. If the proper impedance, you could get them to work, but usually would have "bell tap" when another phone was lifted or dialed.

Back in the early 1980's, we would charge around $80 to do a conversion