RetroGamer4Ever wrote on Today, 14:42:

We need to figure out the technical process for switching an SD-50 to digital output. It is the only module that Roland still supports on current systems, they are readily available for use, and the quality is definitely there, from the game MIDI vids I've viewed on YT.

I didn't found service manual for SD-50, but SD-35 uses uPD6376GS-E2 which is usual 16-bit DAC that inputs i2s bus. We need detailed pcb photo of SD-50 to figure out what DAS is used.
Anyway i think that its 100% possible to mod SD-50 to have SPDIF with my https://github.com/vetal-esher/18bit-DIT