Depends on the old indexing spacing of the freewheel. If it's the same as a 7 to 8 speed then it'll work. If it's Suntour or something else it won't match the spacing.
Also if this is to work without a lot of fussing over spacers then it'll be with an old style 7 or 8 speed. The 9 and 10 speed stuff uses a narrower spacing than the older stuff to fit in the extra cogs. The 7/8 speed stuff is iffy but the 9/10 is a non starter without a huge amount of fussing.
The other thought would be to change out the derraileur and shifter to more modern options. Or just go friction.