head gaskets don't give problems
probably overheated from weed in the water intake, they just slow down and stop when they overheat, it doesn't do any damage as they don't seize,
it could also be fuel, might be stuff in the carbie,
if it was a spark plug it just would have stopped,
How long did it run OK for? and does it run at all now after cooling down?
Before taking on the head gasket I would go through all the usual stuff first, points, plug ht lead ignition back plate screw and ensure the carb is scrupulously clean as is the fuel line and both filters ( tap and banjo - that sounds like a musical act! ) and you are using fresh fuel well mixed to the correct proportions.
If all the above is OK then you could be looking at a head gasket problem, or, more rarely if it is pumping water OK, a cracked block somewhere. Could even be an air leak that 'opens up' as things get hot and expand. Classic symptoms include running for a few mins before stopping but will start and run for a little while again only after cooling down.
Good luck if you take on the head gasket, read the FAQ on the main site and take your time.