Have you tried running the motor a little bit with the glow driver still connected and powering the glow plug? This sometimes makes the difference, and can narrow down problems. If it runs smoothly with the driver still connected and 'ON' the plug isnt glowing correctly.. Im assuming you have tried this..

One other thought, if your running smooth at high RPM's but struggles and dies at the low end, being it has sat for years, there could just be some crud in the pump from sitting, that just wont 'clear out' when running. High RPM's there is enough fuel being pumped/forced to pass, but low end is trickle fuel, not able to pass easy to the needle valve...

I'd carefully tear down the pump and clean it out with some fresh fuel, since you are planning on rebuilding it anyways with new O-rings soon.

Also, might be worth inspecting any gaskets in line with the fuel system.. back plate, etc. We are talking about the pump thats mounted to the backplate of the motor, right?Not the Perry style ossolating pumps.. If its a perry.. bypass that thing for a bit, and run the motor off the muffler's presure for a test.

Also.. seriously inspect your fuel lines for any pinhole's ornicks. easy to not notice them, and can get nick'd easy when installing the tank. they can be damn near invisible.