Thanks to all who responded with ideas. KBH Photografix was able to supply a replacement part, and assured me they had a larg-ish stock of these gears. I bought one, but thought seriously about getting two or three for future use, as this gear is easily stripped if the enlarger head isn't locked down before moving the enlarger or if the elevation motor chassis isn't detatched before moving the enlarger. I think that if I am more careful in the future with moving the enlarger, this gear should last a good long while, even though it is nylon.

Thanks again for all the ideas!