On an unrelated note, if you have the blueprints, how hard is it to get replacement portal parts out of a bog-standard fortress?
((This is probably a question for Mr Frog.))
*Actually, a bog-standard fortress with metal casting, oops.
Probably not a question for me, because I have no bloody clue what you're even talking about, lol ^^; It's pretty funny how out of the loop I am now when I used to be such a thundering know-it-all.
If you're talking about just in-story as opposed to in-game... Mr Frog already built one makeshift portal generator pretty much from scratch with his own hands and knowledge (I think?), although IIRC it already didn't function up to snuff even before Urist "upgraded" it and accidentally made it implode into scrap metal after one use. So, it's definitely possible, but if even Mr Motherfucking Frog couldn't make it work perfectly without specialised equipment and tech then it's probably not possible period, and extremely-inadvisable for someone without his centuries of study and natural intellect.
TL;DR: You could definitely make another portal assuming you had either Mr Frog or someone with a similar degree of technical know-how, but it definitely wouldn't work very well even then.
Also, while I'm here... I'm thinking of doing a succession fort, and I just need some quick input.
If I do it, it'll either be using 34.11 with some minor mods (the Holistic Spawn and a modified thralling syndrome in which the thralls are actually practically-killable) or the old 2D version. The current version is right out due to the fact that from what I've heard sieges are nearly non-functioning, which should make for a boring fort at best. I'd just like to get an idea of which version people would be more interested in.