Reddy heater R50B --- plug into 110V , spark plug fires continuously, motor does not start but seems to spin freely by hand, plenty of fuel, cleaned photo cell (also by-passed photo but same results)

These heaters have a lockout that has to be reset. If it doesn't see the flame with a sensor, it kills the pump so as to not pump unburned fuel. The sensor and lockout are in one piece as I recall and has a reset button to re-arm it. Your user manual should show this.

Things I would check in order:
when you plug it in and it "hums" does the fan rotate? if so does the ignitor fire? remove the top cover and look at the fuel nozzle while running, should be an even cone shaped spray pattern. if no spark remove the ignition lead and hold it close to the end of the ignitor, check for spark there, you should be able to slowly pull it farther away fron the ignitor up to about an inch(use insulated pliers) cause if it bites you it stings, if not the ignition module is going bad. if you have a weak fuel spray remove the back plastic cover on the motor and there are filters inside, then remove the six bolts that hold the inside of the filter housing to the motor. inside is your pump, should be a round carbon rotor and four pump vanes. make sure that there is no damage and also check the plastic coupler that the rotor attaches to the motor armature. if everything looks good re-install the pump and filter housing. like in the previous post check the fuel hose and the hose running from the pump to the fuel nozzle for cracks. you need a pressure gauge that is in 1psi. increments and install it in the port on the back of the pump housing that does not have a hole in it. run the heater and check the pressure, it should be around 5psi but the pressure setting should be listed on the side of the heater. to adjust the pressure turn the plug with the hole in it until the pressure setting is correct. as long as everything checks out the problem is usually solved. depending on the model heater there are photocells(flame sensors), high temo sensors, and thermostats that could hinder the function of the heater. these are just some basic troubleshooting steps for forced air kerosene heaters