I got to thinking about this.
My thinking was, if you could connect the serial port of the 200lx to a midi adapter, you could connect it to a midi device; an MT32 or PMA-5 or some other synth, and have audio for it.
The PMA-5 is about twice the size of the 200lx, but will run on batteries...
So I started looking up RS232 to MIDI... I guess it's not all that easy because MIDI uses an unusual baud rate and some electrical difference.
But I did discover that the gameport MIDI adapter on most sound cards puts out TTL on pins 12 and 15 (iirc) and then the MIDI cable adapter translates it into the type of signal needed by MIDI devices.
But, further, I stopped by Adafruit, and came across this featherwing MIDI adapter...
Adafruit MIDI Featherwing Kit
Theoretically, you could connect the RS232 to this device, and this device to a midi synth, and have audio (for whatever games/programs support MIDI), but you still have the baud rate issue.
So... you could connect the Featherwing to this Teensy 3.2 + Header, which has 3 UARTS, one of which could talk to the 200lx (or other device with an RS232 port) and another to the MIDI device. It would take some programming, but I think is theoretically doable.They have other, similar boards which may work better; I dunno cause I'm not very knowledgeable (yet) about electronic circuity like this.
I've also seen posts on making a midi controller from an arduino, but I'm not sure that's what's needed here. I like the Teensy because even with the featherwing adapter, it's... tiny.
Almost as small is making a midi synth from a Raspberry PI Zero 2 W. The setup here communicates via USB, but the Zero W does have UART pins and should be able to talk to the 200lx at whatever speed the 200lx wants.
The nice thing about this solution is that it will work with pretty much any vintage hardware with an RS232 port and software/games that support MIDI. Theoretically, you could even add a character lcd to the device so it would say "Insert Buckazoid" while playing Space Quest (will Space Quest play on the 200lx? I hope so...)