Plus I can't figure out how to remove the old one (it's twenty-six years old).
I used to work maintenance in group homes and I replaced a lot of old seats. At that age it might have metal bolts and nuts, which will be rusted solid. Even nylon nuts seize up. The bolt goes down
through the top with a nut underneath. Give it a good spray underneath with WD40 once a day and there's a chance the nut will free up. Probably not though. If it's a nylon nut you could break it with a chisel. Or use a Dremel. If you have one you can chop away the top of the bolt and it will fall out the bottom. That works with metal or nylon.
Guess what I'm saying is it might be hard. If I lived close enough I would have to figure out an excuse why I couldn't come round and help. Good luck.