Thanks Blighty and Skinnyhouse for your help. It seems that it was my fault because when opening the font paths, I had it with the eye layers closed, so the plugin didn't recognize the option. I'm sorry about that.
It worked it great as it was hoped to do.
Here are two files that you may find of use: The first provides a set of kerning paths for the Champignon style custom font and the second one is a file that contains a set of updated character paths (three more characters) and that also includes the kerning paths.
Been a while since any resources came here but with the new filter (Custom Font Single Path Stroker) came some new character path files. They are compatible with the original plug-in as long as they have the character followed by char - as in "a char" etc.
I did a font quite thin so that the paths weren't too thick
I suspect you did a Text To Path which gave you paths like the one on the left in the image below. That will work, but what you actually need is a path like the one on the right.
Inkscape does not do centre-line tracing, so can't be used. I don't know of a utility that does centre-line tracing, maybe someone here can recommend one. That would produce something like I did for the Hershey Text and can be imported into Gimp as paths.
As for characters that are not on your key-board (like the heart) - just delete them in the paths dialogue.