Force - simple answer - you gotta build a wheel out of that rear hub just to be able to get that cog off non-destructively! And it you are going to do one, you might as well do the other. (Maybe you are lucky and that cog is just hand screwed on. But that would be no fun.)
If I had those hubs? I'd find some 1950s rims, build them up and start looking for an old track bike to put them on. (I'll bet you can find one cheap and they are so simple, the work to make it properly ridable shouldn't be much.)