I think you could add a UI project using Xamarin without much trouble and make the whole thing cross-platform with Linux, Mac and potentially Android/iOS support. From what I've seen the WPF-specific code is restricted to the AlephNote.App project in the WPF namespace. It should not be difficult to reuse the non-WPF code in a separate UI project.

I've been dying for an open-source replacement for Evernote and I've been looking for an opportunity to learn Xamarin so I'm willing to contribute this if you're interested.