The images used to render the scores and timer are located in script\default_system\img\Default_SystemDigit.png, and then it's drawn as a SpriteList in script\default_system\Default_System.txt. You can change it however the heck you want as long as you know how SpriteLists work, or even as long as you can properly manipulate some sort of render object. Rendering other images as pseudo-fonts use the exact same method, but just with more rules per character.
If you want to use that sheet, then go ahead. These aren't fonts, they're just images. How you want to draw letters on the screen is just a matter of "draw these pixels at these coordinates". In that regard, Danmakufu doesn't really have a real default, it just uses scripts like anyone else would.