The aviation world has similar stuff. In the US, the experimental homebuilt scene is great. Lots of us (myself included) buy a kit and build and customize from there. My build is a kit from Vans Aircraft (likely the biggest of such manufacturers), but other companies sell kits and even just plans to build to your heart’s content. As the builder of record, you’re free to build and customize things like engines and avionics without being a certified mechanic.