A 75 should be fine for the power supply. Presumably this enlarger uses a mixing box, so a flood or spot may not make much difference. Offhand I'd expect that the flood might avoid a possibilty of a hotspot, while you probably get more effective light from the spot. I'd probably try the spot first and make some careful test prints. If the color temperature is not the same, it will probably throw the filtration off some. Testing would be needed.
For the socket, I've seen some specialty bulb suppliers on the web that sell them, but I don't have any links. But a good lighting/electrical supplier should be able to get them. If nothing else, go to one of the big-box home centers and get the cheapest MR-16 track light head and take it apart. I'd also try Granger, digikey electronics, or maybe McMaster-Carr.