Having a spare coil is useful and reasonably inexpensive.
However, as Paul suggests, It does sound as if fuel delivery is worth checking again.
Given what you have checked, if the bike ran for a couple of minutes and then subsequently less time, I would be suspicious of the needle jet. If not that, something which would reduce fuel flow to the float bowl.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.