Conduct a survey of the property to determine what information is available. Information about the foundation, the timbers for framing and other material can be helpful in rebuilding. If you are unable to determine this information yourself, you can hire professionals who specialize in historic restoration.
Consult original floor plans and building plans, if available, of the farm buildings. This information is invaluable as it can provide scale information about the building's construction.
Look at photographs of the farm buildings if they exist. When the barn at the Washburn-Norlands Living History Center burned, this was one resource they pursued in the rebuilding project. "We are seeking old photos of the barn as built at 'Norlands' in 1867," the site requests, "to use as reference in the rebuilding process." If you are not able to find pictures of the farm buildings, pictures of ones built in the same era can be of assistance.
Hire contractors who are familiar with historic rebuilding if you are not experienced in doing the work yourself. These professionals will be able to aid in the rebuilding process.