So long as the thread and shaft that goes into the wiper motor isn't rusted, you will be okay.
Once you have a new nut holding it on, mask it off and paint it with a spray can to stop it rusting again, if you think it is necessary.
When my brother bought his U13 SSS, the nut was missing and the rear wiper arm was in the boot. I used a nut off of the front wiper of a hilux to re-attach it (had spares for the wifes Hilux Surf...). Same thread type and same size.
You will probably find that most Jap cares will use the same nuts (similar size and same thread type) for attaching the wiper blades.
Dan.