First off, that chip has a code-protection option. If it is set, you are SOL.
Assuming it isn't, is the chip through-hole, PLCC, or surface mount? If through-hole or PLCC in a socket, you are in luck - remove the chip, plug it into a Picstart and read the code. If you don't see a bunch of FF's on the screen around address 0, you probably have the code and can start programming replacement chips.
If the chip is an SMT soldered to the board, first study the traces and see if the PGM, PGD, and PGC pins (use this -
https://www.electro-tech-online.com/custompdfs/2011/11/30569b.pdf ) go to a separate connector. If so, that will be an ICP (In-Circuit Programming) connector, and you can wire up a cable to the Picstart, power up the board, and read it that way.
If not, things get more complicated, they were probably factory programmed. You are going to have to remove the chip and make an adapter board for the programmer, and the odds of it having code protection go waaaaay up.