Hi,
you don't need to donate at all, you're request is welcome. The sources for DroidReader are available here:
If you (re-)use the source code, please take into account that they are licenced under the terms of the GPL v3.
Note, though, that the project is unfortunately often stalling, due to very, very limited time resources.
But I think the code should show some basic principles. I would even consider the JNI interface quite elaborated, the same goes for the tile-rendering-thread. Just take the code and if you're unsure what some part of it does, introduce some additional debug output to help understand what gets called under what circumstances.