Genuinely, I think your best bet is to keep the domain and just remove the mccodes and replace it with GL tostart learning like you want to. It'll be easier than trying to sell.
If you're worried about players being mad you removed the mccodes version, I am sure you could create some translation functions to move users over to gl.
Mccodes is a pretty dead engine, which is why not many people here use it. It'll be a tough sell for even the initial $50 engine investment. I guess to put it simple buying mccodes in 2024 is like buying a rusted out 1980s mini truck. Not much value to most, but there's a very small market of enthusiasts