Hi Tobias.

The glass is actually plexiglass and is original.

Because it's an unusual shape, you'll find it difficult to get any sort of replacement especially if you're looking for something other than the plexiglass.

A watchmaker should be able to make sure the glass is secured properly if it's loose, and polish out any minor scratches.

Early Heuer serial numbers are notoriously hard to verify given that Heuer themselves don't appear to have kept a record of a lot of them!

Enjoy your watch!

Mark