http://www.avahifi.com/root/audio_basics/1990-11_fm3_keep_working.htm
The stock FM1 is mono. The multiplex module will make it stereo: equal to an FM3. The FM3 was stereo from stock. You might be better off just "restoring" it to new--I don't know if anyone does any actual upgrades--at least anything beyond a better parts swap.