highlux

This Shimano UN51 73x113 resides in my 96 Karakorum. Bike was lightly used before multi year storage.

I read they are non servicable. I cannot regrease. Is this right? Can i run this with factory supplied grease after 16 years?
The bearing is tight. No play. No noises. Spins nicely. What would you do?

If I were to replace it, and for future reference. What canI replace it with? I dont think the Un51 is around anymore.

Thanks in advance.