Just my 2 cents worth:
In my many years in the restoration reconstruction field I have seen various approaches to problems such as you describe,I suggest that you try and purchase used timber, that can be resawn with a band mill. Place the newly sawn pieces of the correct dimension into place with the band sawn surfaces hidden. If you are looking for a muley sawn look then you need to acquire timbers with that particular type of original finish.
This problem of repairs to original structures is not a new one, but one has to remember that repairs did occur to the historic buildings from time to time as they aged due to weather and rot.
For instance the 1867 muley mill that I am familiar with needed siding replaced from time to time as well the raceways were continuously being replaced every 15 to 20 years over their lifetime so a newly inserted board or timber was by all means permitted historically. Another occasional replacement was the shingle roofs that look very new when first applied, but soon ages.
you need to sit down with whoever you are working for and explain to him that repairing with aged replacement parts is not an easy thing to do, or accurate in my prespective according to what really did happen historically.
NH
