Hi,
I'm interested in adding S/PDIF output to some old sound equipment.

There's at least one product that does this, the
'MDC-1'-FM / PCM sound source multi-data converter".
It's a board that connects to certain input lines of the old sound board and converts the digital signal to S/PDIF format, and outputs the signal to a standard S/PDIF plug.
It has a list of supported FM and PCM chips, including:
OPL3, OPL2 variants
C140
C219
C352
YMZ280B
YMZ770C
SPC700
CXD2922 / CXD2925Q
CXD2938Q
TAITO F3 (TDA1543)
SYSTEM SSV (uPD6376)
86 Sound source board (YM3433B-D)
X68000 / X68030 (MSM6258V)

Technical description and store listing in Japanese, translates well with Google translate:
https://osafune.github.io/mdc1_revb_docs_jp.html
https://peridotcraft.buyshop.jp/items/38086232

Priced at about $115 USD

For convenience, here's the product summary translated to English:

• The MDC-1 mixes the digital data of various FM sound source ICs and PCM sound source ICs into a 48k S / PDIF optical output.
• It has two input channels, each of which can receive digital data from individual sound source ICs.
• Since digital data is converted to S / PDIF as it is without using analog signals, the sound quality of retro PCs and game boards can be easily improved.
• Converts the digital signal of the sound source IC into a 48kHz / 24bit S / PDIF optical signal and outputs it.
• FM sound source IC covers DAC interface of major chips including YM2151.
• The PCM sound source IC supports 16-bit right-justified format or I2S.
• Mixing output of two input channels is possible.
• Supports a wide sampling rate from 15kHz to 62.5kHz.
• Compatible with X68000 / X68030 ADPCM left and right PAN.
• The compact board size (38 mm x 70 mm) makes it easy to install in equipment.

Did anyone try this product? Are there any similar products available?