Actually the cranks aren't particularly heavy.
Alloy squares don't overheat.
Burman clutch and gearbox (GB), both excellent.
4G MkII, as used in that special, 42 or 45hp as standard, depending on who you believe. As standard though, it should have an SU carburettor, whereas that looks as if it has amals. Normally the inlet tract goes from the carb to the centre of a cruciform manifold, a rather tortuous path, so changing the carb in itself won't do much good. Perhaps they've constructed a better set-up.
You don't rev out a square, even though it is so smooth; much better to ride on the torque, and you can maintain good road speed averages, much more easily than the 650 twins of the period.