Nice find,
Hope that the reeds are not too rusty!
Are the keys stucks? the hammers move freely?
You can easily test the piano in a passive way, by picking a 1/4 jack out of the knob on the ''harp'' direct to the pickup output (Like a Rhodes Stage) and test out the pickups if the electronic isn't working.
Those missing hammers could be 3d printed i think, dunno if they are hard to remove, never done it!
Good luck, more pictures could be interesting!
Xavier