Rain water with oil
? If your genset has to be flooded with anything that is the best!! The following will take someone with mechanical and electrical skills and it will try tricky to do do it long distance, but give this a try:
It sounds like the generator end was wet to some degree. Have your yard remove the generator, spray all exposed electrical connectors with WD40 or similar water displacing solvent and put it in an oven
and bake at low temperature- 100 C for a day or so. It may be fine after that treatment.
The engine sounds likes it may be ok, but some parts need attention- certainly the starter. Remove it and bake it with the generator. Did water get in the crankcase? If not (and you can easily check the oil
level to find out), try to start it and run it for a few minutes. The oil will look chocolatey brown if even a little water got in it. But change the oil and run it for another few minutes.
Hook up the generator end and see if the whole thing works. At worst it sounds like you are looking at a new generator end, but maybe not even that.