Acrylic Windows on steel boat. How to seal them?

[zeseb]

The previous owner of my boat did a mistake by installing some acrylic windows (8mm) himself with the wrong screw material and different sort of sealants. So it leaked at several places.
I started to remove all the sealant around the windows. Some screws are still stuck in the rust. I need to find a way without breaking anything.
But some screws were not even screwed in the metal frame of the boat.
So by pulling a bit the window, I discovered that some part of the steel frames are fully rusted. I suppose a bit of welding to replace some part of the steel frame will be necessary.

Meanwhile, I'd like to redo those windows properly and use the proper products.
What sort of screw material shall I use, and what sort of sealant and where shall apply them.
Do I need to put some screws?
Any ideas will be useful and if any piccies examples, I take it.

Thanks!