It's a cause for celebration when I find a completely original piano, including the hammers. (The hammers and finish are usually the first things to be replaced, followed by the strings.) Technicians will usually reshape the hammers to eliminate the grooves. Conservators prefer to send the original hammers to Europe to have the wooden moldings recovered with new felt, retaining as far as possible the weight and resilience of the original; however, this still destroys the original hammer as a historical document. I prefer to replace the entire action so that the original action can be saved intact for historical study. More often than not, however, the client cannot afford this, in which case I prefer to replace the hammers with new ones of similar weight and density, along with the shanks and flanges, and save the originals for the historical record.
Steinways built from 1864 to 1880 usually have actions that appear modern, but have a much higher action ratio (leverage). These are cumbersome to play with the heavier hammers of the modern era, including modern New York Steinway hammers, so I encourage the use of lighter hammers and higher gearing on Steinways originally designed that way. I also retain original rocker-style capstans instead of retrofitting with new screw capstans and new-style wippens.
Today I replace soundboards less frequently, and when I do, I copy the original design, extensively document my findings, and retain the original soundboard as an artifact. I also retain any unique design elements, such as the angled mid-treble unison terminations used on Steinway Cs and Ds from 1884 through the 1930s, and faithfully replicate them if a bridge must be recapped. This unusual element adds sustain to the treble by slightly de-tuning the unisons. Squaring the termination to tame the resulting slight "wildness" of sound neutralizes the designer's intent for the treble tone.