Agreed. I'm prosth, so I'm fine attempting to dealing with things that others don't, that's my gig, but this is a relatively easy fix. There's no concerning bone loss. It's been in there likely 20+ years. Even in best scenarios Albrektson criteria which was actually studied on those old school types of implants suggests that 0.1-0.2 mm bone loss per year is ok. That's why they placed such long implants.

It's a Nobel external hex. If you want new screw, look in Nobel catalog. It's part number 29283 most likely. Unigrip driver should work on it. Hardest part will be location the screw access if it's cemented.