Moderator: OpenTTD Developers
Download and follow the usual steps for compiling source code. If you have any particular issues, please feel free to post them here and somebody may be able to help.
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1. OpenTTD is written in C++, it's over 300,000 lines of code.
2. It doesn't use the GPU for rendering, drawing is all done by the CPU. You'll have to re-implement that from scratch. Be sure to keep all functionality of NewGRFs, so likely you need to implement some part of NewGRF at the GPU.
3. Once you did that, you'll likely run into performance problems. The current code is highly tuned to run a lot of vehicles smoothly. A JIT interpreter with automagic garbage collection like what's below C# is going to have a hard time keeping that performance.
So good luck porting, you'll need it.