Some auto parts stores carry metric drain plugs. I have seen them as big as 27MM. It helps to have a similar plug in hand when trying to match one up. Will any of the other plugs on the machine(gearbox, hydraulic resovoir) fit that hole?
Failing that, you might be able to use a rubber expansion plug(available at most hardware stores) temporarilly untill you can get the proper plug ordered. Most expansion plugs use a center thread and nut assembly you just want to make sure that any threaded portion that is inside has the threads at the end mashed over so that it wont drop the nut into the gearbox if it should vibrate loose.