Welcome! It's good to see a fellow frugal recordist.
Unfortunately, I've tried one of the $5 eBay USB-to-MIDI cables such as Mark linked to, and it's prone to stuck notes.
The M-Audio UNO is about the least costly I can think of:
https://www.amazon.com/M-Audio-UNO/dp/B00007JRBM/ If you think you might ever want to record audio, you could get one of Behringer's interfaces that will allow you to do MIDI as well as audio for not much more. Their quality has been getting better and better.
You can get surprisingly decent audio playback with the on-board Realtek HD chip, either using ASIO4ALL or WASAPI.
Cool that you rescued the Casio from the dumpster. She may repay you for that kindness. Rescue instruments can be like rescue dogs in that way. I loved my Casio, it had a fast, springy velocity-sensitive keybed and some cool built-in rhythms, like the "Blue Monday" 16 Beat. There are some good sounds that come with CbB and
tons of freeware and inexpensive softsynths out there.