While it is correct that FritzingCheckPart.py currently modifies the file, it leaves the original file in filename.bak (because the error message line numbers refer to the input file, not the pretty printed modified output file.) Running FritzingCheckPart.py twice (without renaming the .bak file) will destroy the original so saving a copy of the original is the safest bet.
For hole sizes the drill.txt file will give the in size of all holes (although finding out what hole it refers to can be a challenge!) If you are using Inkscape as your svg editor the hole size is set by hole size = pad-diameter - (2 * stroke-width) So for an svg at the recommended scale for a .1in header which wants an 0.038in hole, with a 20 thou ring thickness, the pad diameter wants to be 0.078in.
0.078 - (2 * 0.02) = 0.038 and the gerber drill.txt file will have a 0.038in hole specified.
Feel free to upload the non working .fzpz file and one of us will correct it so Fritzing is happy with it and repost it for you. There are a lot of constraints which check part tries to deal with but sometimes it misses (and since I wrote it, I can usually figure out what is wrong.) There are some Camdenboss parts in core parts so exporting one of those as an fzpz file (right click on the part and select export part) then unzipping the fzpz file will give you the files of a working part to use as an example.
Peter