The first thing you need to do is make sure that what you are looking to use is definitely public domain. Some stories maybe PD, but some characters might not be, Plastic Man, the Spirit, and Gene Autry are all good examples.
As a writer, I try to look for characters that are completely PD, study everything that we can possibly know about them, and then look for a new story to tell about them. In many cases, I like to reveal some new revelation about the character, and apply it to the pre-existing material. In that sense, I've now tweaked a dozen classic characters or so in different stories that I have written.
There are a number of different ways to find out just how PD a character is, most of the information can be found online, and trust me, it doesn't take a lot of effort to find this info out.
My only real question to you is this, why try to copy something pre-existing? Why not take a PD character and write something new featuring them?