You can buy an adaptor for the Spotmatic that will convert a current battery to the same voltage as the mercury 625. Although most 42mm lens are interchangable you do need to be careful. Late model Fuji Yashica's 42mms (designed for AX body) do not mount well and will not couple with standard 42mm bodies. On the rear of the standard 42mm lens is a pin that controls the aperture to close it down when metering or firing the shutter. When looking for bellows or extension tubes look for the pin. The Spotmatic F meters at full aperture, as does the ES. Fs and ES that have motor drive written on the base plate will take a motor drive. GAF (Chinon) made a body with shutter priority exposure like the ES, and Chinon also made a body with a motor winder (very slow 1.5 to 2 FPS) but sounds like a sewing machine. Cosinga (sp?) makes a current 42mm body. You can also get a 42mm to K mount adaptor and shoot with most K mount bodies with most lens in stop down metering. I have a ME with an adaptor, also a Nikon 42mm adaptor, but my 28 and 24 mm don't seem to meter with the ME and 300mm and longer cut off the corners on the Nikon. Most electronic bodies from the 70s (including the ES) are failing and unless you have a body to cannibalize will be difficult to repair.
All said I have gone though many Nikons, (F, F2, F3 FE and FG) but the Spotmatics keep on trucken along. Great basic body with excellent glass.