If the frame has 130mm rear drop outs, a rear mech hanger, down tube braze ons for stops or levers and you know what all this means then fitting any derailleur groupset should be possible.
If the rear end of the frame is less than 130mm you won't get a cassette hub to fit. If the frame is steel, the stays are long enough and the spacing close enough you could cold set (bend) it to the new size.
If you've got long forward facing drop outs but no derailleur hanger then you can find a detachable one from a (very) old mech and use that.
If the down tube has no stops then you can maybe find band on ones or get them brazed on if the frame is steel.