Good question! One option is to do some research on the FSK [Frequency Shift Keying] process and see if that is standard for all cassette data devices that use FSK or if each manufacturer used a proprietary method. If it is standard, maybe there is a general FSK to data program somewhere. If it's proprietary, maybe the CZ-5000 service manual might help.
Another option is to record the FSK audio into a wav file [don't use mp3!] and either post it somewhere or e-mail it to someone who can play it into a CZ-5000 and play the midi data in realtime into a sequencer that can save to a format you can use, possibly a standard midi file.
If you have a dual tape deck or two cassette machines, I suppose you could make a dub (turn off NR) and send that to someone, but I think creating a wav file would be easier to share.
In any case, if you have an index to what's on the tape (or in the file), that would probably help whoever will be doing the conversion.
There may be other options; these are off the top of my head. If you create a wav file and post it somewhere, let me know; I can probably test a sequence or two to see how much time it would take. If it goes fast, I can probably do a whole bunch.
There is at least one other option--have someone load the data and then do a sysex dump of the sequence. However, then you'd have to write a program to convert the sysex dump into a standard midi file. Depending on the number of sequences you have, it's probably not worth it to write such a program.