We can't assess the condition of the exhaust valve without either doing a compression test or removing the cylinder head and inspecting the valves. If you were doing the latter you would lap them so you knew they were sealing properly. Normally, though, even if the exhaust valve threw off a spark, it wouldn't make it through the muffler. Most likely any spark that comes out from the exhaust pipe was generated in the muffler, by burning old carbon. However if you are going to run that old engine under load, it should be checked out first.
That begins wth some form of compression test. Preferably, use a pressure gauge with a non-return valve so it retains the maximum reading, and hold it against the spark plug hole while cranking the engine rapidly, with the throttle wide open. If the pressure is less than 100 psi, you should then put a spoonful of engine oil in through the plug hole and repeat the test. If it doesn't increase to perhaps 120 psi, you would need to remove the head and lap the valves. If it does increase to a decent figure, the valves probably are in reasonable shape, but the rings are worn out.
