When a painting is out of copyright, making a copy isn't an issue. It's ensuring it's clear that it's a copy that's important, as you don't want anyone to think you're passing it off an an original. So don't copy the original artist's signature, and put a note on the back of the canvas that it's a copy of X's painting by Y. That way there's no doubt as to the origins of the painting.
Whether you sign the front or not is a matter of personal preference. I would sign the front with "After XYZ by ABC" because then, in addition to it being clear to someone's looking at it that it's not the original, they know the name of the contemporary artist who painted it should they wish to have their own copy.
For tips on how to copy Old Masters, the materials to use and modern equivalents, the Art Forger's Handbook is very useful. As you'd surmise from the title, it's written by someone who used to sell fakes, and thus really knows how to make a good copy.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.
