If you have traditional portlights, it's usually possible to remove the hinge pins and take the frame holding the glass off of the port. You need to to this to replace the glass, anyway. On most traditional bronze portlights, there is a bronze ring holding the glass into the frame. The ring is held in with tiny machine screws, which are often under the gasket
Either take the whole assembly to your glass shop, or disassemble it and take them the glass. Automotive safety glass works well and is easy to find. Clean everything thoroughly to get rid of the old sealant
. Put it back together with your favorite bedding compound to seal the glass in. I've used silicone, polysulfide, and others, with varying results. I would not use 5200, as someone will eventually want to take it apart again.
It's not a hard job. The tough part is usually getting the old glass out. A thin, sharp, flexible blade applied with patience works, but the best way is to use a Fein multimaster with the flush cut sawblade if you have one handy. It's certainly easier than replacing the whole portlight. I've done my dozen ports
more than once, as the safety glass tends to delaminate after some years, clouding the view.
Good luck with your refit