I've got the original WMC 20hp carb sitting around in the basement, if it does you any good. I can check the casting # if you're interested.

I don't know about any modifications swapping from Tillotson to WMC, Boat_Head's description is certainly detailed enough to follow if it's right, but finding the reed block might be its own issue.

This is the carb I had originally:

https://www.boats.net/catalog/mercury/ou...-12-24-25-26-27