The problem with replacing the Suntour Accushift 7-speed freewheel is that the Suntour Accusft freewheels do not have liner spacing between the cogs. At one end of the freewheel the cogs are farther apart than they are at the other end. Changing just the freewheel or just the shifters may cause shifting problems with the derailleurs not moving the correct distance with each click when in index mode.

I don't think the special Suntour Accushift freewheels have been made in years, and the only way to replace it is probably to look on eBay. Or Craigslist to see if you can find one.

You could try a cassette with standard spacing and see if your shifters and derailleurs will work. I bought a Trek 1100 with Suntour Accushift, that had a bad rear wheel. I decided to change over to a cassette rear wheel and eliminate the Suntour components to eliminate any chance of not being a being to find parts in the future. I experimented replacing one part at a time to see if the resulting mixture would function.

The first thing I replaced was the bad 7-speed rear wheel with the Accushift 7-speed freewheel. I installed a new Nashbar bar rear wheel with a Shimano freehub and Weinmann rim. It seems to work okay with a 7-speed cassette and a spacer (rear hub is a 8/9/10). Next I switches the downtube shifters to Shimano CL-R400 8-speed. The rear derailleur did not have enough range to shift all 8 speeds, so I set it up to shift 7-speeds and it worked. Last, I changed the front and rear derailleurs to Shimano 105 9-speed parts and they work just fine with a 8-speed cassette and the 8-speed downtube shifters.

You can do something like what I did. Replace the damaged Accushift freewheel with a 7-speed freewheel with uniform spacing between cogs. If that does not work, you will have to replace the shifters. If the bike is worth the trouble, or if the rear wheel needs replacement, just swap to a freehub rear wheel that will take a cassette, and see if the shifters will work with the cassette.

If you have to replace the shifters, you can consider going to brifters (at considerable expense) or change to 7 or 8 or 9 speed downtube shifters that will work with standard parts. You can get Sunrace downtube or stem shifter for between $25-$35, or used Shimano 7-speed shifters ($5-$50), or new Shimano 8-speed shifters (about $40)