Barreiros, motor humano
It's an AJS, British. AJ Stevens & Co. From the look of the tank and look of the top end, it's Pre 1935 but the picture doesn't show enough detail to be able to tell exactly the year/model. AJS made a V twin, looks like this. 1932 plate.
Nearest match I can find for the exposed valve gear, AJS TB8 Big Port.