The firing pin and bolt sleeve run around $35; the conversion cocking piece and spring would bring maybe $20 if anyone can be found who wants them. At one time, that conversion was common, but with the rising prices of original military rifles, not many folks want to lower the value of a 1917 by altering it.
FWIW, the conversion changes the gun to fire off the firing pin retraction cam, reducing firing pin fall to less than half its normal distance. That is why the spring has to be heavier than the standard spring. Even then, misfires were common with that conversion, especially with hard military primers.
Jim