You can replace the entire thing with a compatible and programmable module, if people care enough to develop one. There are now cars from the 80s that are modern enough to have computers, but old enough that the ECUs are starting to deteriorate, and still appreciated enough that people repair or replace them.

Example: (not an endorsement and I have no experience with this product)

http://roguetuning.com/rogue_tuning_ecu

You can swap out the entire thing if you want and do a reflash. It's not a beginner job but it's also not impossible, or illegal (if it still meets emissions, but that's on you)

hearsay from my car enthusiast friends and this cursory internet search contradict that claim.

https://duckduckgo.com/?q=modify+ECU&t=ffab&ia=web

Yea maybe you're right and you can only partially modify it.