Disconnect the gas line at the carburetor and check for a full stream of gas from the tank. Use a clean container to catch the gas. If there's just a trickle of gas your gas tank needs cleaning and the gas line blown out using air pressure. If the flow looks ok from the tank remove the tube fitting where your gas line connects and look for a screen plugged. If there's no screen pull the carburetor and soak it in carb cleaner and blow out all the jets.
One other thing to check is the fire to your sparkplugs. Pull off a plug wire when it loses power and see if you have a hot blue spark to your plugs. If it's a weak spark install a new set of points and condenser. See if that improves the power. If it doesn't replace your coil. Hal