Good or bad an old farmhouse is drafty as I'm sure you know. The best thing is to combine the proper amount of attic ventilation with insulation, so the insulation stays dry. The first step is to add 15-20 inches of insulation in the attic. Lay it across the floor joists. This will make a huge difference in your comfort.
To add some additional ventilation, add gable vents if they don't exist. Gable vents are not as efficient as ridge and soffit, but gable vents plus the drafty house should provide enough ventilation for the insulation to work properly.When you need to reroof, be sure to add a ridge and soffit vent system for ulimiate vent efficiency.Another insulation option is to consider spray foam - products like Icycne are perfect for old farm houses. It can be directly applied to the underside of roof sheathing. In this case the attic is not required to be vented and the spray foam insulation will insulate and seal drafts at same time.