[Kerbiter]

(It's not a reimplementation, it's a different thing that is asset-compatible with the originals. The gameplay, the balance, the engine are different, and the name of OpenRA is not fairly claimed IMO.)

[Maakuth]

I would imagine OpenTTD also has a number of similar "rule tweaks". The OpenTTD engine AFAIK started as a direct reimplementation, but I think that too has evolved quite a bit.