It's likely any RD, probably 8 speed and under will work with those shifters but they are unusual, they appear to have Cyclone era levers fitted into Diacompe Silver end pods? I assume there's no ratcheting mechanism in them. You'll need at least a med to long cage to reach the Mega Range freewheel.
You could easily upgrade that very fine frame to 7 speed indexed or not. I think it's a 126 rear spread. 8 speed generally requires 130 mm. Or ride it as it is.