Your first step is to make sure it is a 7 speed freewheel and not cassette. I'm assuming it is a freewheel.
I am also assuming you have Suntour Accushift Plus downtube shifters or Suntour Accushift Plus thumb or X-Press shifters.
Here is some interesting info Suntour freewheels and cassettes...
Archival Reviews of Suntour Freewheels/Cassettes - the Buyer's Guide, 2015
And Sheldon Brown's Cribsheet...
https://www.sheldonbrown.com/cribsheet-spacing.html
If you are able to unthread the first cog and separate the cogs from the freewheel body, the older ones allowed for this to swap out cogs, you can measure the spacers to see if the are all 3.15. If they are, they you should be able to use any Shimano/Sunrace 7 speed freewheel and it should index.
If they are not, here is an option...
https://www.ebay.com/itm/SUNTOUR-FRE...sAAOSwgbte1Ahh
I would contact the seller for a P/N off the box.
You can always change from index to friction and use the Shimano/Sunrace freewheel. Or you can buy Shimano 7 speed index shifters form $20 to $80 (eBay).
And there are probably people on C&V subforum that have better info.
John