The best open source alternative to Typora is Joplin, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Typora and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Typora are Mark Text (Free, Open Source), Zettlr (Free, Open Source), ghostwriter (Free, Open Source) and Trilium Notes (Free, Open Source).
Buho is a note-taking app. Buho will let you take quick notes, save links, create books and pages for taking classes notes. It is free and multi-platform, and open source under the LGPL v3 License. Index is made with Mauikit.