This is fowl pox, which is a viral disease. It is spread by flying insects like mosquitoes. There's no treatment for the disease itself, but to make it spread slower, treat your area for mosquitoes and flies. Also, isolate the infected birds.
To make the bird more comfortable, you can put petroleum jelly mixed with sulphur on the scabs. This will soften the scabs, and you should be able to remove them. The sulphur will help repel flies.
Though the disease is viral, a secondary bacterial infection could form due to the stress that the bird is experiencing. Because of this, you might want to get some antibiotics. Terramycin or Tetroxy HCA-280 are two good antibiotics.
There is a vaccine, so think about vaccinating in the future. Birds that recover will be immune, but can still transmit the disease.