Discontinued sure. But the used market can find some deals.
Are there any parts that would be an issue assuming I make no modifications?

I moved on from mine because it was heavy, and there was no aftermarket support, or parts availability from SIG. But yes, they shoot well.

^^This for me as well^^

You could just shoot it until something breaks (which should be thousands of rounds) & buy another complete upper to replace the piston upper with.

The only parts you would have to really worry about are the piston parts. The bolt carrier, I believe, is interchangeable with the LWRC carrier. Think Sig was sued because they were basically the same when the 516 came out. I have another Sig 516 upper on backorder with Optics Planet. Will it ever ship, your guess is as good as mine.

The one I have is heavy as hell, but shoots better than I can.

I have two 516s that are very reliable. One is heavy as hell as stated above. And one is lighter with a 14.5 inch pinned barrel and a Lancer handguard.

The 14.5 inch upper which came with a buffer spring and buffer was sold by SIG for $600 plus tax and shipping. Had to have a good smith do the pin and weld. I built the lower with the lightest parts that I could find that are durable.

The 14.5 inchers came with chrome lined barrels.

Bill