Most cams are advertised by valve lift and total duration. There is no way
that I know of to measure duration without putting the cam in the car and
measuring with a degree wheel. Maybe you could do it out of the car but I
doubt is since the exact position of the lifter is crucial to when it
starts moving. Also, any degree information taken directly from the cam
would have to be doubled since it spins half as fast as the crank.
Valve lift is related to cam lobe height by the rocker arm ratio. I don't
know what an MG's rocker arm ratio is. A small block Chevy ratio is 1.5:1.
so .100 lift at the cam = .150 lift at the valve. In mechanical valve
lifter cars such as an MG you must also subtract valve lash. So valve lift
= (lobe height - valve lash) * rocker arm ratio.