shell script for removing unprintable characters in file names

Hi,

I am tasked with recovering hundreds or more files created with unknown 
OSs and have unknown characters in the name, replaced with a '?'.

Like file?nam?.???

Please, if you have such a script can you post or email it? Replacing 
the unknown character with anything, like '-' or '_' using whatever 
shell, sh, bash or csh.

Thanks a lot!

Per