@wyleu Are you sure the 6N139 is going to work just like that? The H11L1 has a schmitt-trigger after its receiver transistor, while 6N139 just has the “bare” phototransistor as its output. Also, looking at the schematic, it might work if you connect the 6N139 on pins (2,3) input, and (8,6,5) output side; that’s a different pinout though from the H11L1, so you wouldn’t be able to just pop it in place, ignoring two pins.
Dug this up because I’m finding myself in a similar situation for my potential 2nd Zythian 