Universal Motor as a magnetic brake
The idea of supplying a fixed DC voltage to the field from a separate power supply seems like a good plan. The original machine very likely had at least the rated current marked on it. The motor seems to have the voltage marked on it and perhaps also the power. There may have also been a diagram of the internal control circuits on the back of the machine. Since the field would have been connected in series with the armature, the field current would be whatever the original motor current rating was. From that and the field resistance you can figure how much voltage is required to produce the proper field current. It is probably best to operate it with no load on the armature to start with. Then measure the voltage output and figure out the load resistance from that.