It has like 50 lines of code that are actually platform dependent, from what I can see.

Should be straightforward to port to GTK# or Eto.Forms or something, if one had the inclination.

https://www.mono-project.com/docs/gui/winforms/ might allow this to be ported easily though.

winforms on mono never really worked, and hasn't been updated in about 12 years https://github.com/mono/winforms

While I'm a novice - Looking at this, it doesn't appear to require any porting.