Pulling a hatch completely from the boat and resealing, at 18 years old, is a very good idea... and, honestly, one that is overdue - although you may not know when it was done last if the boat is new to you, which of course, is another good reason to do it. On the plus side, it's a simple job.
Oh, I mistakenly mentioned Life Calk above. While that might work, a better product for that application is the Life Seal. You can also use Life Seal for the hatch-to-deck resealing, too.
Resealing around the hatch... Cut a V-groove out of the existing stuff with a new razor blade, then come back and trim some more of the old stuff away. You don't have to get it all. But, sure, you could completely remove the acrylic.