Stolen from Wikipedia:
Cirrus Engine was a British aircraft engine manufacturer, known for their line of 4-cylinder air-cooled inline engines for general aviation use. The company was bought by Blackburn Aircraft in 1934, and operated as a separate division until production ended in the post-WWII era.
Cirrus's first product was the 90 hp (67 kW) Cirrus Minor, which passed its 50 hour type rating in 1925. It was the first air-cooled inline engine, a design that proved extremely popular for light aircraft. The basic layout was quickly copied by a number of other manufacturers.
The Minor was known for excellent reliability, and had a major "win" when it was selected to power the RAF's Auster observation aircraft. The RAF's version had several modifications, known as the Series I. Although externally identical, the Series II engine was redesiged to operate on 77 octane fuel, as opposed to the original's 70, increasing power to 100 hp (75 kW).
And even more amazingly you can buy an 18cc replica of it for your model!!
Enjoy your Austering! And look after that antique dragging you airward!