How to remove "custom" paint work without removing factory paint

Hi Joe, welcome aboard. Be very careful with Simple Green. It is a strong cleaner but it has been known by experienced folks to cause serious corrosion problems on electrical contacts, as well as metal components like track and terminals.

If the cat walk is removable do that and using a soft brush and some Tide powder in warm water softly remove the ugly paint. Tide powder is good for getting Transmission fluid and engine oil out of concrete so it might work for this. But be careful I am not a detail and paint expert at all, so I don't speak out of experience only from "How Too" videos I have seen.

In any case be ready to do a full repaint, Testers is a good product and dose not come off easily.

Remember what ever you do wash off you work with cool water when done the cleaning process, most chemicals do not stop working just because they are dry to the touch.  

Have fun and good luck.

I tried toothpaste and old toothbrush of course once and it worked.