Some of the early Casio boards used analog drums but they switched to PCM samples fairly quickly. I just looked up the board on Tablehooters and found this at this page: http://weltenschule.de/TableHooters/Casio_PT-80.html
"simple analogue percussion with transistor noise (base, low tom, mid tom, high tom, snare, open & closed hihat}"
So it looks like you're correct, they are analog. Personally I would still sample, please don't kill any more of these sweet little keyboards, but it's up to you of course. Have fun!
On a similar note, if you're trying to explore electronics, you might try building your own simple drum machine with individual circuits for each drum voice, it can be very simple with just a few parameters per voice, and it does not need to have a sequencer or anything like that built in, just a MIDI interface to trigger each voice. I recommend this over modifying the Casio, and you'll have better sounding drums too!!!