I agree with Peter.
Releasing a new version of a game engine takes the development team months or even years. They have all the original tools, documentation, and experience with the game engine, yet it requires much time and thousands of lines of coding.
I don't mean to hurt your feelings, but have you estimated the amount of coding that you must search in a game engine? In the range of 100,000s to 1,000,000s of lines of coding must be searched and parsed.
Added to this, there is the issue of the range of coding to be examined, too. There is low and high level coding, as well as branches. Truly only an expert in said game engine, working with a team, could hope to complete a new version in less than a couple of years.
Everything that you need with a game engine can be accomplished by simply scripting a game source code of your own. The game engine might allow Python, C#, Lua, Java, C++, or whatever, and most of them are designed for a particular workflow pipeline of software and applications for most efficient development.
Go trekking into the jungle of a game engine alone if you like, but it is best to follow the proven and beaten path made for the beginner.