If your bike has the SunTour accushift compatible freewheel cogset (early 1990s?) and indexing SunTour shifters, then you must stay with SunTour derailleur to maintain the proper relationship between shifter pull, derailleur travel, and cog spacing.
If you have thumb shifters running in friction mode, then it doesn't matter - any derailleur that is rated for the overall capacity range [(front outer teeth - front inner teeth) + (rear inner teeth - rear outer teeth)].
So, what are the make(s) of your shifters and rear cogset (freewheel or cassette)? PG
PS: I use an X-1 on my road touring bike, as it's perfect for the wide range gearing. Reliable, functional, and inexpensive. Why not simply replace it with one from an eBay seller? They can be had for <$30 if you're patient...