This is what an exfont file looks like. You would need to have an image that size, called exfont, but saved as a bmp file.
Each icon is 12x12 pixels, and it can only be black and white.
When you have it, download the program ResHack. The downloads topic no longer has it so I guess someone else can link to it if they have it. You open up the RPG_RT.exe from your game in there. Go to Action -> Replace Bitmap
In there, go to "Select bitmap to replace" and scroll down until you find exfont. Go up top where the button is, then click it and find the new exfont file. When you have it, go to Replace, and then go to File -> Save.
That should be it. If you want, you can back up your RPG_RT.exe file if you're worried about making a mistake or something.