The Suntour freewheel is 30 years old, not going to do 7 speed SIS index shifting.
(Cogs and spacers too wide). But quality is as good as Shimano.
LBS trash many 7 speed wheels. Cheaper to replace than labor to fix.
Chop off outer 3 cogs and weld it up. Free wheeling function not required.
Freewheel must safety wired to prevent it being unscrewed.
Exploded 7 speed freewheel:
Homemade SS Freewheels - step-by-step photo guide
Specialized Fatboy 20" has 4" tires, disk brake and 9 speed on 170mm spacing,
(same spacing as OCC ?) :
Specialized Hub 170, Hi Hi disc, alloy, QR, 32h
Sub $200 Mongoose Fatboys might be the same.