I would suggest contacting Shell to see what they recommend as a substitute. Another company worth trying is https://www.morrislubricants.co.uk/ I use their oil in a number of my machines.
I am not familiar with Kohler engines but should they be bolts or studs?
Looking at a copy of the manual it shows studs. If this is correct I would replace the bolts with studs and nuts
I am not familiar with the detail of these machines but a clear photo of the tensioner set up might help determine if it is a later modification, it is not clear in the first photo.
A good question and an interesting subject for someone to research if it has not already been done. I wonder what reply you would get from Honda if they were asked?
There is a page on Wikipedia but no information on tillers.
There is all sorts of information on the main Honda website. I found this reference to their first tiller
Photograph of what you have in the magneto would help but it sounds like and electronic ignition device has been fitted to replace points.
Damaged thread is a bigger problem and will probably need a new crankshaft, again a photograph would help.
