In fact I dont think the beta with no patches will even run on modern windows
It easily does run. All you need is the compatibility mode (XP SP2).
Use megabuild, theres no real reason to play an unpatched beta.
Leaknet is far from ideal (it wasn't really finished), not to mention you can't really add anything, like backporting mechanics and whatnot. Plus, it's not exactly the same engine as the playable beta, the leaked source code is slightly older and has more quirks. (so would any custom compile though)
Sadly, I wasn't able to get it to work either despite setting up XP and Visual Studio 6.0 and everything and compiling without errors. For some reason the shader dll just wouldn't work right with the engine.