You don't need the fancy mounting nut - just use wire ties or similar and mount where convenient. The only important spec is mfd, the voltage can be anything over 250V.
NextGen sells Phasor also and you could probably buy a new head
from them or elsewhere. They may be able to give you some trouble shooting advice. It would be unusual for the generator head
to be a complete loss given its age and the way it suddenly stopped working without any other symptoms.
We have had both Phasor and NextGen generators. The Kubota engine parts
are pretty universally available, as are the standard marinization parts like the water
pump, zincs, etc. The generator heads used are either Markon or Stamford and are used worldwide in lots of construction generators.