Can't you use a spdif to I2S converterboard?
Something like these? (no experience)
https://www.aliexpress.com/item/1005003382164642.html
https://store.sure-electronics.com/product/AA-AB41133
There appears to be a bit of analog filtering and gain that happens before the PCM1808, and the clock of the PCM1808 is sync'd to the internal crystal that's running at 256fs. It's possible, but I don't really NEED it to be better.
I'd rather just buy a proper TimeStep T-03EQ instead of re-engineering the Puffin! Less effort, just money...