Suntour Accushift rear derailleurs are not compatible with Shimano-compatible shifters. These derailleurs require about 10% less cable pull than the Shimano units of the same era, so the shifting will be seriously off, and most bike mechanics (even pros) will not understand why.

If you hope to keep the XC LTD, which BTW is a fine derailleur, then you'll need to find an Uber-rare matching Grip shift right shifter. Good luck.

But if your shifter is salvageable, I would replace the rubber and call it a day. Replace all cables and housings of course: a yearly maintenance necessity.