Original as in F&CC #20?
Based on what I have seen them working on, and what was done by the D&RG before it went to the RGS, probably only parts of the frame, parts of the boiler and the driving wheel centers and boxes as well as the cylinders and saddle. Possibly the sand dome and stack (but they could have come from either 22 or 25 at some time as well as both were cannibalized for parts to keep 20 running). The tender is from the 25, the pilot wheels are almost surely the second or third set. The pilot truck has been completely gone over at Strasburg as well as most of the boiler, and I cannot believe that there were not both boiler and firebox repairs between the 1890's and 1951. The frame has been completely rebuilt at Strasburg as well, so there are some old and some new frame members, and again, how much of that frame that made it to Strasburg is original is a big question as well.
I am sure that with the meticulous records that Kelly and Lynn have kept, that they can truly answer the question when they are done, but it will be a long list of what is rebuilt, and a short list of what is original especially to the F&CC #20, and longer list as to what was RGS 20 in 1951.
20, like most existing steam engines today, is our version of grandpa's axe. 2 heads and 6 handles over the years, but it is still grandpa's axe.