It is indeed amazing that one can buy one of these original blocks and have it drop into apparently any Peabody receiver without any need for hand fitting! The Providence Tool Company certainly had the capability of producing interchangeable parts in quantity back then!
One of the conversion methods I saw (and may have forwarded to you) had a coil spring fitted over the firing pin, but I don't have one on mine, and it seems to work just fine with the fairly broad pin, with its quite rounded tip.
Both of the rimfire Peabody models, as well as the other centerfire versions, were designed without any sort of "retraction spring". The hammer should not be left down on the rear end of the striker when opening or closing the action, so the striker (or firing pin) will be free-floating .... and just gets cammed back as it passes across the rear of the cartridge. If the hammer is left down on the striker, the strength of the mainspring will keep it pushed forward - not a good thing!. However, it is unlikely that any retraction spring which would fit in the firing pin recess could possibly be strong enough to overcome the force of the mainspring, anyway ....