you can't fit a single piece crank through the much smaller bottom bracket of a 3 piece, be it cottered or cotterless.
If the Puch Free Spirits weren't british, Swiss, French, or Italian threaded (I want to say that they're not), you can go with a standard bottom bracket and get a pressed-on square-taper bottom bracket that has been the standard since the late '60's, but honestly, there isn't much of a reason to go with anything other than the original cottered bottom bracket in this case.