Experiments like these are really fun to do in a small setting. In reality you would only get to this point after having built a complete programming language with all its parts. But in a situation like this you can start at the end, focus on generating the machine code, putting it in a memory buffer and instructing the CPU to start running it at that point. It can hardly be called a JIT compiler of course, but you definitely get an impression of its mechanics.

A couple of years ago I did the JIT 'kickstarting' machinery for fun in the Nim language: https://github.com/eterps/loader/blob/master/syscall.nim