Hi Marioxp,
This is my first post in this forum. Since I bought my boat (similar age than yours) a few months ago I've been reading this forum and learning a lot from their members and then applying this knowledge to my own case.
Regarding your question I can tell you my own experience. I had two types of problems with my hatches: crazed acrylic and one of them had broken the gasket.
So, after reading and watching a lot of videos I decided to buy the gasket by meter.
To join both ends you can use Superglue-3 with activator as Yngmar has said but that's not the problem.
The problem is finding the right length of the perimeter, because if you calculate the perimeter as the sum of the sides the result is very "bulky" and you won't be able to insert the acrylic into the frames and you will brake the gasket as I did.
On the other hand if it's too short the gasket won't fit around the acrylic.
So unless someone tells you exactly the distance I will recommend you to try to fix the old gasket with super glue if you just need to join the ends or other type of glue made out of rubber if it's a hole or a crack.
I don't know how to send a picture yet but I will try to learn to show you my experience.
Good luck!