Hi, I'm no expert with these engines, but I checked the records for an O & R 1 Hp Chainsaw model from early 1960s and it lists ratio as 16-1.
Now I have had discussions with the Chainsaw Website owner in the past and he reckons that if you have an engine that has a Chromed bore, then that Petrol/Gas ratio can be increased safely to 25-1.
My 72 Beaird-Poulan Chainsaw has a Chromed Bore and I use that ratio, as well as classic 40wt Mineral Oil (to avoid any issues with synthetic additives affecting the old Oil Seals).
I also find that 25-1 is mentioned for these O & R engines on U-tube examples.
As for a suitable Oil available in your Country, I found this example is basic Mineral with no Additives- OIL
API-SA grade is devoid of Additives listed here-
I'm sure there will be a fellow expert Countryman along at some stage to offer guidance, as I'm on the other side of the Pond.
Hope this helps