A few years ago I bought out a school's old 404s. They had been stored for years and most of them were frozen up like you say. I learned a lot from messing with those machines. Rodney is right old oil will need to be dissolved. But most of those machines had 'frozen' motors. What worked best was to remove the motor and put one drop of oil on the bearings, then rock the shaft back and forth until it turns real sweet. You might do well to locate a service manual so you can feel more confident about pulling the motor. It is a very easy job.