In the 0.2.2 release, zep made a small tool for creating custom fonts:
https://www.lexaloffle.com/bbs/?tid=41544 (search for "Custom Fonts")

It's a great tool, but if you only have a font snippet (and no longer have the spritesheet), it's hard to edit that font again. So, I extended the tool to also support font importing!

You can get my font tool from the pico-8 console:

LOAD #FONTER

Instructions on how to use the tool are in tab 0 of the cart. happy fonting!