If that rear hub is toast, and it's an actual Suntour freehub/cassette system, well.... punt.

If you've found a GPX rear hub that takes a freewheel spaced at 126, you can put a 6s or 7s freewheel on it, no sweat.
Threads are more universal than slots/splines, and a heck of a lot easier to find mates for.

If you've gone the freewheel route, by all means use a Sunrace or Shimano or Sachs/Maillard.
Anything not "straight-cut" will shift better.

With Suntour indexed, when that chain settles on a cog, you know it.
It's not a mystery. It's not a maybe. It's a fact...."thunk."

I'm running a set of GPX hubs with a Shimano 600-level 6-sp freewheel.
The rest of the bike is Suntour Superbe Pro and it's money.

I have another set of the hubs, but I'm quite sure I promised them to a guy for a decent lunch.