Excellent Track and Sector Editor.
XVI is a very useful Track and Sector editor. Open the file you are interested in. Then you can search for a string of hexidecimal characters or ASCII characters.
You can change the hex or ascii to whatever you want to. If you keep the number of characters after making changes to the same number the program will still function but the displayed text will change.
You could also take chunks of code out from inside the program as well. Very useful program.
Very simple and useful.