Rear tyre in 140mm (same size as on a 50cc 2T I had 20 years ago), where I put a 160mm on mine.
I would have expected a 150 or 160.
At least here in Italy, but I bet in France and in many other countries, too, the vehicle registration certificate lists the allowed tyre sizes and usually there is more than one; hopefully, they allow bigger tyres to be used.
They also use a 4.5" rim according to the Cycle World's article, which is a smart choice IMO. I tried a Michelin Power Pure 2CT on a 4.25" rim years ago and I liked it; I thought that probably a 4.5" rim would have still fitted without problems and would have worked better...
On Stark's site they have written that the SM shares the same frame of the MX 1.2.







