I hesitate to get involved with something with so many variables but here goes. First check with local shops and see if one has an old engine analyzer. This will show the condition of the entire ignition system and a good tech can even read valve and cylinder data. If I were to take a one shot guess it would be a weak coil as these will perform good cold and drop off rapidly when hot. A stuck heat riser blocks the exhaust gas exit whether hot or cold so I would put it low on the list. Rig up a one gallon jug for a fuel supply and temporarily run rubber lines from jug to pump inlet and pump outlet to carb. In the carb line use a "T" and hook a fuel pressure gauge to it to check for drop off. This eliminates the fuel tank and lines as a trouble source. Start cold and let the carb bowl fill and measure gas height in the bowl. Run till it "drops off", turn off ignition switch, throw into neutral and coast to a stop. Pull carb top and measure fuel height. If lower than the cold reading the pump is weak and you're starving for fuel. Rather doubt this as usually a lean condition will cause back firing. Re-read the first line as an analyzer will give a reading on the coil, wires, points, condenser, plugs, and a good tech can see rotor and cap spikes if he is familiar with the engine. This does away with buying a bunch of unneeded parts by trying to out guess the problem. I wish I could be a swami and tell you to go directly to "this", give it a twist with a screwdriver and all would be well. We don't guess at anything but rather eliminate till the problem is found. Not the easy way or fastest but the problem WILL be fixed and done without wasted money on unneeded parts.
