The original speaker would have a field coil on it to generate the magnetism to make the speaker work and to act as a filter choke for the power supply.
If I was replacing the speaker I would use an 8 ohm permanent magnet speaker. The field coil can be replaced by using a filter choke and a power resistor. The combined resistance of those two should approximate the resistance of the original field coil. On a 20 I believe that was about 3100 ohms. A single resistor may be used but a lot of heat may be generated. Alternately another field coil speaker may be used along with a resistor to approximate the field coil resistance.
I am not sure of the voice coil 8 or 4 ohms, perhaps some else can comment. Also a member may have spare field coil speakers to sell from other radios.
Ed.