Buy another 6 speed freewheel. and buy a new chain ,
the possibility is that the chainrings have worn significantly also,
but they can be replaced..
though you are looking at a significant budget to do all that..
but all are mechanically renewable.

dont have the money? pull the freewheel off, and soak it in solvent, like Kerosene,
It may be gooey dry, then once it breaks free, run some 90 weight gear oil in.