I managed to work out the resistor network parts. RP1 and 2 are actually 16 pin dip packages, and are there. RP3 and 4 are the single in line type, with RP4 being a regular 10K 8pin network. RP3 is also a single row of 8 pins, but each pin doubles the resistance of the last, so 10k, 20k, 40k, 80k, 160k and 320k. How on earth do I find one of these? I look for resistor networks, and the only ones I can find are ones with a single value on all the pins.
The only other passive part I am not sure about is R41. I cannot find it on the schematics anywhere, so I am not sure what value it should be, but I suspect that it may not need to be there, as the few pictures I can find of the board do not show it being populated. Oddly, on my board it has obviously been cut off the board (the holes have solder, with leg remnants in them), so maybe it was a factor fix for something?
For those interested, here is the sheet I made with the entire list of parts of the board, from the schematics and verified with the board I have in front of me (though the schematics do not list the right chips in some places for some reason).
Google sheet link
So this leaves me with working out how to get an IWM and a COPS41. I have lead on the COPS, but I have absolutely no idea where to find an IWM. Is there a part number someone can give me to search for? Is the IWM just an EPROM with code on it, or is it more like one of those chips you program once and you're done (PGA type thing)?