The nice thing about Accushift 6 speed is that it's the same spacing as SIS, so a standard 6 speed freewheel will do ya- and the Shimano will be inexpensive and better in most every way for indexed shifting.

The "alpha" Accushift systems have a different cable pull, so you're limited to mating @ shifters with @ derailleurs- but the freewheel shouldn't be an issue as long as it's a standard spaced 6 speed freewheel.