Apparently the way to find amputation miracles is to use a relevant verb (like “amputavit” (although I think there are other verbs for when bits just fall off, and I’m not sure how to search for miracles involving people born without something)) and a relevant verb in the accusative (like “pedem,” foot, or “pollicem,” thumb).
First off, St. Julius of Novara was a real early Christian guy (4th century and the tippy-top of the 5th), and his brother St. Julian the deacon was a real guy too, according to archaeological finds at their traditional burial place in Gozzano. They are better known in Italy as San Giulio d’Orta and San Giuliano.
Their Vita says that they built one hundred churches during their lifetimes. Those of us who live in US dioceses where there were guys tasked with building churches for population booms will not find this number too farfetched.
According to their Vita, the first church in Brebbia, Italy was built by them, and that’s where this miracle took place. Unfortunately their Vita was written in the 7th or 8th century, so it’s not well documented; but hey, it was reliable on the burial site. (And of course there were times when a vita was based on local archives as well as legends and oral tradition; and oral tradition isn’t necessarily wrong.)
From the “Vita SS. Julii et Juliani.” Collected in the Acta Sanctorum, Januarii vol. III, January 31st, p. 718.
Also, another miracle happened at the place which was called ‘Beblas’ [Brebbia].
When they had set about the work [to build the church], one of the men amputated his thumb, having unexpectedly put his hand ahead of time on an iron tool which is called in the people’s tongue a “hatchet” [dextrale]. And so much gore flowed forth from it that the man lost his reason. The common people associated with St. Julius were eager to point this out to him.
So the holy Julius, coming to him right away, searched for that same thumb, saying, “Bring the finger to me which was taken off.”
Having received it, he put it into its place; and after making the Sign of the Cross, it was restored as it was before – an entire hand [restituta est, sicut antea fuerat, integra manus].
And taking the iron tool, the man of God gave it into his hand, saying, “Work and take courage, in the Name of the Father and of the Son and of the Holy Spirit.”
In Italy, San Giulio is the patron saint of builders and stone-masons. The feast of Ss. Julius and Julian is on January 31, and there is a great Mass held on Isola San Giulio on that day, which is attended by many people in the building trades. The builders traditionally give an offering of a lamb decorated with ribbons, which is then blessed, roasted, and feasted upon by all the visitors. (Isola San Giulio isn’t a very big parish, so it’s a case of Bring Your Own Dinner. The picturesque bit is that they actually get to bring the lamb into church to be offered and blessed, whereas your average feast featuring a blessed pig/cow/goat roast has to keep the livestock or ex-livestock outside.)