Wow! You could probably just get some sort of sound module for voicing and leave it at that, but my suggestion is to bring it up to date with a decently equipped computer as your recording device (DAW), which will also host any sound producing software (VST). Then you will need to buy an audio interface to which you connect the MIDI IN/OUT of your KX88, and the KX88 connects to your computer via USB.
Are you familiar with these devices?
It's been years since I've played with hardware synths, so I can't get too involved here on that level. However, there are lots of others here who can chime in with words of wisdom.