If you are happy with the sound why would you risk mucking it up by switching the drivers?
Considering the un-availability of large ortho drivers in "raw" state* (to say nothing of how they might sound - the fact that after some 15-20 tries between hifiman, audizee, and Fostex the various ortho designers have come up with 15-20 versions of the best, and you can bet that the next one will be even better!) why not just love these for what they are and buy a set of (insert the name of the headphone whose sound most closely matches your ideals here) already built and without the mystery of what might happen if the drivers you find happen to suck?
Orthos can be built that are so bad they cant even form a cohesive apology for how much they suck. Dynamics can be built that surpass electrostatics by any metric you choose. Electrostatics can be built that sound exactly like orthos which sound exactly like dynamics. Its not hard. There is no "driver sound" Well, there is - Grados sound like Grados. Stax sound like STAX, Sennheisers sound like sennheisers. you get the idea. Not because they are dynamic, or electrostatic, or ortho, but because when they were put together they happened to have a certain sound.
On that note, I think some of the older Fostex headphones had drivers that could fit that headphone. It would not be very hard to cut the hole to be MUCH larger than it is, and then you could certainly fit them. Is the driver board MDF? Just a few minutes with a coping saw would get any shape you want.
* By this I mean in order to get Ortho drivers you essentially have to buy a headphone and take them out. The tweeters people used to use are nearly impossible to find. And also more like 40mm so too small anyways.
