It seems replacement remotes for your unit are hard to find, but
here a guy describes a hardware hack allowing use of "any" Sony remote.
I am not surprised to see yet another proprietary Sony technology into an (expensive at its time) product - my mind can't accept the fact that on the acre of front panel space, Sony did not put simple up/down/store buttons for FM control.
But the leadership of "we know better than the dumb user what he/she needs" has long been taken by Apple
😉