Well, you could scoop out the internals of your 200 chassis and slip in a pi zero W, an oled screen, etc. it would be some work but there’s probably no reason you couldn’t pull it off. You’d need a different (and better?) battery solution than AA, but it’s really not hard to get your pi to run off battery and take usb-C charging. You might enjoy reading these links:

Somebody did just this with a 95 LX. https://liliputing.com/raspberry-pi-brings-an-hp-95lx-palmto...

The details page from the guy that did the 95 LX: https://hackaday.io/project/172230-hpi95lx

People here talking about similar ambitions for the 200LX https://news.ycombinator.com/item?id=7055384