Honestly? It's shockingly plausible, if you threw out the shooty parts of this game. Limiting it to a door-sized height, on a wall, would be a little wonky, but without the enemy grid there's plenty of RAM for saying "portal goes from here to here." How it's done now is a frankly ridiculous hack with a tiny array of "special tiles." Shooting the portals so they fritz out moves the special-tile reference a few steps way, and then it advances every few seconds until it finds a matching level tile. Which allows some bizarre level gimmicks where a shattered portal reappears somewhere else.
That Portal 64 port is slick as hell, though. Dude's polished it halfway to being the preferred way to play the game.