When I used a Mainstream Cooper engine in a competition car build I got a lot of conflicting information. The engine had to be technically 'standard', so I needed to carefully build it by 'blueprinting' as far as possible.
When it came to it I could not determine which cam was standard. Yes, there were lots of opinions from other 'experts', but in the end it seemed that ARG fitted whatever cams they had in stock. Then the valve sizes were conflicting. When I stripped the original engine it had 33.5 mm inlets, but the Haynes Manual quotes 35.6 mm.(i.e. MG Metro size).
The conclusions I came to were that ARG fitted whatever they had in stock and I had this confirmed by an ex-ARG purchasing person who said that they were, at that time, not paying their suppliers on time and that deliveries is specific parts, like cams, could be delayed, so tin order to keep the line flowing they fitted whatever they had 'on the shelves'.
In the end I bored it to 1330 cc, as permitted, used an MG Metro cam and head, skimmed the head by 0.027", the allowable standard service skim being 0.025" (a scrutineer could not measure to within 0.002" and that would be an allowable tolerance anyway). I flowed the carb and cleaned the casting flash from the manifold. The head ports were 'improved' slightly, but not so much as to be noticeable if inspected. I fitted a Mk.1 Cooper 'S' rocker shaft and Aldon customised a dizzy for me. With a few other 'tweaks' it gave 84 bhp at the flywheel 5700 rpm.
It is hard to be specific about any Mainstream Cooper engine without stripping it down.