I am going to tell you how to flash the field. The worst that can happen is this is not the problem.
There are 3 ways to restore the magnetism that I know of.
1. (This is the best option) Jumper 12V from a 12V battery, positive to + brush and negative to the - brush. Hold it for about 10 seconds. Start the genset and see if it is putting out. If not, jumper the 12V again with the engine running. This time only hold it for about 5 seconds.
2. I have only read about this one - I have never tried it, but it is the recommendation from Briggs and Stratton. Plug a reversible drill into the outlet, start the genset, put the drill in reverse, squeeze the trigger, but manually spin the drill forward several times by hand. The theory is that spinning the drill the opposite direction of the switch creates a small magnetic charge in the motor that is sent to the windings of the generator, creating the magnetic field.
3. This one was given to me by an old-timer at Kohler generators, but you msut be extremely careful. (This is actually the one I use) Get an old power cord or extension cord (with the female end removed) about 4 feet long. Cut the cord about one foot from the plug and install an in line fuse holder in both the hot and neutral wires. Put a 10A fuse into both holders. Plug the cord into the outlet. Hook up the positive wire to a 12V battery. Start the genset, and touch the negative lead to the negative post on the battery. Pull it back off in 3 seconds (count one thousand one). If the genset has taken a charge, one or both fuses will have blown. The theory here is also that you are inducing a magnetic field into the windings.
If these do not work, there are other problems with the genset.