Can I do that?
If you’re building from source (GitHub). Sure. Not sure what else will fall out from that change but it should be okay, and that’s the only way.
ExtraLifeMatt above explained what i mean 
you also need to move header file to Public.
Remeber that editing engine kind of ruins idea of plugin, because plugin user will also need modified engine in order to use your plugin, you could more effectivly add module from you plugin to the engine itself direcly