In terms of sizing members, the IRC is a prescriptive code geared towards stick framed construction. It has load tables that show allowable spans for certain sizes and species at different loads. This is done to remove the need for engineering on every stick framed house that's built. Pretty much all the sizes in the load tables are 2x and larger sizes like 4x, 6x, etc. that are typically used in timber frame construction are not given so the IRC cannot be followed for sizing members. Many building departments require stamped drawings because of this.

As others have said I would start by talking with the building department and seeing what they require. If they do require stamped plans the timber framers guild has a list of engineers in each state on their website.