Pretty much like this. Doesn’t look like there is much on the inside at all. Power stage, control panel interface driver, MIDIBOX programmed module, and the FM chip board. Lots of info and awesome sound samples are located here.
http://ucapps.de/midibox_fm.html
It also turns out that those FM chips are easy to obtain from old soundcards like the SoundBlaster Pro. Thanks to Phil for showing this to me.