How to write an engine using a custom rasterizer and pure C in the best way possible?

We can't tell you.
Why not? Because we don't - can't - know enough about your project, or your abilities, or the environment your engine will run under, or how it is supposed to communicate with external apps (which the job of an engine after all!). We don't know how far you have got so far, or even if you have started anything yet. Heck, we don't even know what kind of game engine you are talking about, let alone what you consider "the best way" to write code! "Best" is a subjective thing: it could mean "fastest", "smallest", "most flexible", "easiest to maintain", or any of another dozen possible criteria.

All we know is what you have told us: "I want to write this thing in C" - but to be honest if you want to write a game engine, that's not a good start: it lacks a lot of really useful features such as object orientation which make your job a load easier.

Sorry, but you are going to have to sit down and work out exactly what you are trying to produce (and why; there is a lot of wheel reinvention potential here) before you can even begin to ask that question of yourself, let alone people thousands of KM away who only get to see exactly what you type!