The bike's a Half Step Plus Granny.
A medium length,,,derailler cage at one time could handle a 28 tooth freewheel.
A 5 or 6 speed 14-28T 0r 14-30T freewheel used to be pretty standard on many bikes. It depends, if if said derailer,,dosen't have a position,or B screw or barrel adjuster a newer swap would improve the bike. Chris