Peter Brueggeman is right that you can just drop the outermost cog on a Regina FW, but there is one cautionary note I'd add to his suggestion about a lot of cog swapping. That is, the Regina, Atom, and others of that ilk (a) thread on all the cogs; none are spline-fitted like Suntour, Shimano, and even the CycloPans of the era. (b) the two innermost cogs are left-threaded.
Without a special tool, it is pretty hard to remove all 5 cogs. And, the cogs tend to get seated very well. Like real tight. Like so tight that I have warped large inner cogs trying to pull them.
So, if you want to work on Reginas, keep your expectations modest... harvey sachs mcLean Va.
Peter Brueggeman wrote: Erik Reese inquired "...Looking for any 3 or 4 speed English threaded freewheels..."
Older Regina five or six speed freewheels (Extra, G.S. Corse etc models) are cost-effective four speed freewheels. Remove the fifth cog (counting up from the spokes). That cog threads into its neighbor 4th cog and does not thread onto the freewheel body, so its removal leaves you with a four speed freewheel. You can assemble a custom freewheel by purchasing a number of such freewheels (which can be inexpensive on eBay if you bide your time), and swapping cogs around. English threaded Regina freewheels are identified by 'FI' scratched on their back facing the spokes, or a single circular groove there.