On my 1926 Fordor, what is correct for the nuts and bolts holding the pan to the bottom of the engine block
Are the bolts and nuts unpainted, painted or cadmium plated.
Are cotter pins authentic for these bolts
Are the bolts that hold the timing cover painted or unpainted or nickel plated similar to the head bolts
The only way to actually be sure would be to see an original unmolested car, and they just don't exist. The nearest I have seen was the 1917 touring Rip Van Winkle that only had 26 miles on it. I saw it when it had 100 miles when it was on display in Bakersfield.
The bolts on the engine were not painted and not plated.
I have never seen an unmolested 1926, but I would guess that they too were not plated or painted.
As for cotter pins, Ford always used the castle nuts but Bruce McC indicates in his book "Car that changed the world" that Ford started to use lock washers during 1926 and of course dropped the cotter pins at the same time. I don't know the exact date, if there was one ever recorded.
From what I've read here, fasteners on the T were never plated unless they were decorative ones (like the steering wheel nut), but they weren't unfinished either - they had a "raven finish", which sounds to me like the modern "black oxide" finish which does prevent rust to some degree, especially if a light oil film covers the black oxide.
Were the cotter pins unfinished, "raven" finished, or other?
Most all of the bolts were raven finished. Since your engine pan bolts are already on the car, I wouldn't worry too much about it, unless it is apart. As far as the nickel plated bolts, the only ones I'm aware of that were nickeled are the head bolts, and the water inlet and outlet bolts.