The display looks like an OLED, the sort of thing you'd find in a car radio, or an LCD with a blue EL backlight. Either would be similar enough for television. The lettering "HIGH EX" is seven pixels high and the suggestion elsewhere that it's a 144 by 32 pixel display is not unreasonable. Someone more dedicated than me could probably find a reasonably high res still and confirm that by counting dots, or at least by counting fuzzy blotches. Do we have an accurate set of dimensions for the size of the display cutout? It is of course not necessarily the case that we're seeing the entire display.
These displays are not, as far as microcontroller work goes, particularly difficult to use. I would avoid the Crystalfontz one as, while they're very good and extremely easy to use, they're a bit bulkier than a bare display. Even a bare display would be extremely difficult to fit in there with a working glock, although if you were willing to partially disassemble the LCD device you could strip it down to the glass plate, the backlight, and the polariser. The polarisers do scratch incredibly easily, though, so you might want to throw in at least a layer of acetate over it. That could be extremely thin, but I still don't think there's any way of fitting a working display in with a working airsoft gun. The EL backlight would require a (tiny) inverter and the whole thing some form of power supply. I dunno, though, maybe you could cram it in the front "magazine".
I'd write the code on an AVR, because that's what I happen to be used to, but there are a number of ways it could be done. This is not something I have time to do pro bono, unfortunately.