This is a strange, specific question to ask, but I can't think of a better place to ask it other than leddit. Seeing as I don't have an account, here goes.
On my birthday, I got a novelty model clip-together kit for a V6 engine from my sister. It's a piece of shit but I built it, let it run for a minute or two, left it in a corner and forgot about it. It's about 20cm on its long axis for scale.
I got to thinking about it afterwards and I had a look at some of the design and engineering principles behind IC engines from sheer boredom, and I started to wonder what would happen if I tried to build an engine of a similar size for an RC car or something just for the hell of it.
I don't know the first thing about engines other than the theory I picked up after building the model. I have access to a CNC machine to cut the components for me from stainless steel, but I honestly don't know if the idea is even feasible, which is why I'm asking here.
Is there anything completely obvious that would stop an engine from being 20cm in diameter from running properly if it was built the same way as a full-sized engine? Why do people not build them and put them in RC cars?