Here are four charts that have been developed from original factory drawings,the first shows RPM against MPH in second and top gears, it assumes that the standarddifferential ratio is in use, and applies to all FWD models. (From drawing DM439)The second, third and fourth drawings show how an aluminium cylinder head affected power output, especialy at higher revs.
The four cylinder three-wheeler was the only BSAFWD vehicle to be fitted thus, butthe title of the drawing refers to this as being a TEN HP engine!.(From drawing DM442). 
This is the power curve for the V twin engine
And this the curve for the RWD ten hp engine