bike wrench

Context: My 90s MTB still has its original Shimano chainring (crankarms say Alivo). The cassette was replaced several months ago, as was the chain and bottom bracket. No wear on the chain right now.

The derailleurs are Alivio, and probably the original ones from the mid-90s. I don't have any reason or intention to replace those, although, I do have a Deore LX on my decommissioned 90s MTB that I could likely drop in, but I digress. LOL

I guess two questions:

1. What would I need to provide to my LBS as far as measurements to get me a replacement? I understand that I'll likely need another BB and maybe(?) a chain, but I plan to stick with the same gearing.
2. Ideally, I'd want to find a 3x chainring where I can replace the crank arms and individual chain rings when needed. Will this be easy to find, considering the age and group set?

Thanks in advance.