Is this bike setup with indexed shifting? If not, any old 7-speed freewheel will work as long as the biggest cog isn't too big. If you have decent calipers you should be able to measure the spacing of your current freewheel and compare to Shimano (all should have the same spacing.)
No problem going from 13 to 14T on the smallest cog.
The old SunTour is long gone and I'd be very surprised if anything remotely close to what they used to make is being produced today.