Another alternative is to look at something like PicoRV32. There's no "OS" as such that can run on PicoRV32, there is only a few kb of RAM, so single C files that you write from scratch. But it's a simple project and fun for hacking. You can put it on a cheap Lattice FPGA using a completely open source toolchain. A fun way to get into hardware / FPGAs for sure.