I think that will be a Mayfield 10, the Museum of English Rural Life at Reading have the factory records and should be able to date the machine from its serial number.
For equivalent tyre size there are a number of online conversion calculators
I think it is worth noting that I think this topic refers to the split wheels as used on Monrotiller etc, not the split rim type of wheel as used on commercial vehicles, which can be lethal.
I would say it is from somewhere in Europe, France, Germany etc. Possibly Bungartz, Siemens, Stuab. It might be worth having a look through Michiel Hooijberg’s website
I don’t have any information for the JAP engine. The workshop manual for Villiers Mk 10, 12 and 15 gives a cylinder head bolt torque setting of 220/230 lbs inches. The crucial part is to tighten them evenly.