I am a retired maintenance worker with a machine shop in the garage. I have not created an engine yet, but am entertaining the idea. I will be watching your projects for ideas.
Welcome to the forum! Saying hello from Findlay, OH!
How much machining experience do you have? That may help us give you better suggestions for a project to work on. That way we don't suggest anything too simple or complex to start with.
I have done minor machine work...making repair parts, shafts mainly, for 30 years. I have a nice lathe, mill, surface grinder, drill press, heat treat oven, sand blast cabinet, variable speed belt grinder, tig welder, torch set-up.