How To Find and Replace in Filenames?

Hope that was a good description of what i want to do.
I have an external hardrive that I did a backup using a windows machine and a different external. Glad i did because it crashed and now copying back to a new external. In the process I have noticed that:
I have a lot of spaces in filenames and maybe or maybe not in the conversion some have turned into funny hieroglyphic characters where the spaces were.

So I would like to find and replace all spaces on an entire drive with the underscore.
Then I would also like to figure out what the funny character is and replace it wiuth the underscore.

Does finder only find? or is there a hidden feature like this?

I do remember doing something like this in Unix a million years ago but can't remember if it was one file at a time, or one subdirectory. I have burned a few thousand of my cd's so need something to work all the way through a directory.

Thanks for any help or any leads.