You can plug a guitar to the input as there is enough gain to get the level - I tried it with a little electric lap steel guitar.
Tom Whitwell also designed the Electrophone which has a guitar pickup mounted on the panel.
It may or not be something to try - Check out his vid: https://youtu.be/2JwwlLcU29s
All the Modular Music Thing stuff is opensource with scematics etc on github.
Good luck with the build