I looked a little further and found an article from Byte (April 1981) which contains quite a long technical article on a design for a data storage system based on the CM-600, starting on page 66 and split up by 100's of ads.
The complexity of the system described there is much higher than the simple "audio recorder used as mass data storage" used by hobby computers like Sinclair Spectrum etc. where data is simply dumped in one lump and retrieved in one lump. For instance, it allows blocks to be written on the tape for searching etc.
However, it does also imply that the CM-600 is simply a pure transport mechanism that specified no particular data format, meaning that all the electronics etc. for the signal encoding/decoding etc. is specific to the application.
I guess if your synthesiser did things like writing a block on the tape, rewinding, checking, rewriting bad blocks, fast searching etc., I image that you'll have trouble duplicating all that logic.
What is actually damaged anyway? Is it the tape drive mechanism, head, motors etc. or the electronics or even availability of tapes ?