Let's say you're a multi millionaire and have loads of money to blow, is there a market for a custom car/engine? I always loved the idea of making a custom car but rebadge it as if it were a production car. Like if I had lots of money, I would love to make an inline 5 evo 6-like car but brand new and to my liking specs, could you commission a group of designers/engineer and CNC machine a brand new engine block and chassis? I imagine it's way easier to take an existing chassis and modify it to your needs and as for the engine I imagine taking an inline 6 and chopping a cylinder is easier?

What would be the correct way to approach it? Would it even be road legal?

There are dozens of highly reputed, highly skilled custom engine builders in the world. I'd recommend you spend time reading and learning about engine design and operation, in theory and engineering practice, so you actually know how to communicate your needs properly.
>road legal
If it's road legal it's road legal. Depends on your local+state/province+country laws. At least for me yes I can make a one-off, fully from the ground up custom car and have it tested for regulations and if it passes, it gets titled and licensed.

>>28483794
Something like a Factory Five Racing Cobra or Roaring Forties GT40 would be unreal.

>>28483732 (OP)
A completely unique bespoke engine is a lot of work to get right. Mostly what you see is these smaller companies copying an OEMs homework to get most of the engineering right, and then modifying the platform in order to fix problems or to add strength in order to handle more boost. Something like the Elmer Racing billet M44 in the RP968 time attack car.

I'd do something retarded like tell them I want a Chevy smallblock v8 but double that sucker so it's a v16 and then shove it under the hood of one of these bad boys, then I'd get an overkill AC system and a Nintendo 64 and go on road trips driving wherever the hell I want in it