Fonts are found in Data\Menu\Fonts and are made up of two files. One is a TGA (or DDS in SH4) that has all the characters lined out as bitmaps, and the other is an OFF file that contains a list of rectangles linking the actual character to a section in the TGA. You can open the OFF files with S3D (v0.8.2 and up).
To make a new font, edit a TGA first and replace each character bitmap with a new one. Then, tweak the rectangles in the OFF file so the game knows what character is where in the TGA. Take note that when you change a font, you likely have to do it in multiple font sizes, ie.: FPHunt12.off, FPHunt15.off, etc.
You can also add new fonts (instead of overwriting) but then you have to also add a reference to it in the menu_1024_768 file and update all the UI objects that have to use the new font.
Example:
