The hub should have a 1 3/8" 24tpi thread which will accept any standard screw-on freewheel block but 3 or 4 speeds are tricky to get hold of nowadays.
5 speed would be easier to find but the spacing of the bike won't be wide enough for it. It can be done with modifications but personally I wouldn't mess around too much with what must be a really old frame.
I assume you have a Cyclo derailleur (or similar non-parallelogram mech)? It will be designed to use a 1/8" chain and as far as I know they all used clamped on gear shifters. Derailleur gears weren't as universal back then and most sports bikes would have been fixed wheel as standard.
The Cyclo gear system worked well enough with a bit of practice but setting one up is a bit of a black art and the gear range is limited.
Google Cyclo-Benelux gears to see what it looks like.
If it was my bike and you just wanted to use it rather than have it completely original, I'd convert it it to Sturmey Archer hub gear as it will work better and probably have a wider gear range.