Decompiling generally doesn’t yield very good results. The decompiled code won’t be pleasurable to work with, even just for reading, especially if the code was compiled with optimisations turned on and debug information stripped.. I’m not familiar with how the C# compiler works internally, but if it is any similar to C then most variable names will be lost and loops may be expanded (where possible) for performance.

Why must you decompile it?

You can, but as said above, decompiled code is hard to work with. Consider rewriting the engine. If you’ve learnt anything from last time then this time should be much quicker, and maybe better.