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Re: Porthole sealing question(s)
In theory, you could use a glass bit, or masonry bit, or fake one by using a brass tube and loading it up with abrasive to grind through, making a hole to relieve the stress. And then perhaps thoroughly cleaning and drying the crack with alcohol or another "no residue" solvent, and applying crazy glue to bond and seal it.
That will still be weak, because the "tempering" is like case hardening, it relies on evenly applying tension to the entire skin of the glass, in one piece, with no cracks or other localized issues. The glass will never have full strength again. Tempered glass has to be cut to shape and THEN fired in an oven to heat temper it. Which is why most glaziers will not handle it, they don't have the glazing ovens so they can't temper the glass.
If you call around, if you find someone who will make up a new window for you, the best bet is to carefully re-glue that crack (for strength) and then carefully remove all the trim and sealant, and give them the old window to make up a new one. You may find a lot of glaziers will say "I have to go now, my mother's calling me" and run away when you say "tempered".
Yes, you can make up a new acrylic one, but I doubt the color will ever match the other "windows", and acrylic will eventually scratch, again mismatching the glass ones.