The air pump is supposed to supply air to the exhaust ports where it mixes with the searing hot gasses. The oxygen in the air allows any remaining unburned hydrocarbons in the exhaust to be consumed in the flame & therefore be prevented from polluting the atmosphere.

Have a look into your carb & quickly open the throttle from idle to wide open [engine off] You should see a nice steady stream of fuel from the accelerator pump. If you see little or none, then this could be the cause of your 'bog' problem.

It's possible that you will need to dismantle the carb for a thorough cleanout & for this, a minor overhaul kit would be needed.

Did you replace the points, then set them with a dwell angle meter before setting the timing, ... or does it have electronic ignition?
Either way, you don't mention setting the timing, so if this was overlooked, now would be a good time to time it.