Agree that the down tube failure mode is not buckling in the middle - this part is generally in tension. Any crash sufficient to buckle that tube means you have more problems than water bottle holes. I'd remove the bottle hardware and inspect the holes, looking for cracks. If no cracks, the bike might be rideable as is.
For a more significant repair, evaluate the frame. Is it a hand-made lugged frame with high-end Columbus tubing, like a Specialissimo? Or a more unremarkable unspecified frame, like a Premio? (Assuming that the bike is indeed a Legnano).
If the frame is worth it, one could sand off paint, and silver braze small coupons of metal over each hole. But this would require a decent framebuilder and would be costly. And it would require paint touch up.