What are some of the obvious pitfalls with a project like this?
First of all unless you are an experienced machinist you are up against a steep learning curve. We generally recommend building at least one if not a few simple air/steam engines to build the skills and build the shop. Then build a simple single cylinder Internal combustion engine then move to the Multiple cylinder ones.
You will probably need to build a machine to grind your crankshaft and your cams.
Learn to create parts to tight tolerances with a smooth finish. If you can not hold a tolerance on one part it will be difficult to make 8 or 10 parts the same. I say ten because spares are not a bad thing and if the last operation on the last part goes sideways you have another one to work on without going back and starting by cutting a piece of your bar stock.
It can take years just to learn the skills to start a project of the complexity you are talking about.
Bear in mind i'm no cad wizard so getting this on a plan of sorts will take some assistance. I know a guy who might get this in pixel form
The more complex the project the more important the plans. You want to know the size and shape of each part before you cut metal. You want to find an eliminate any errors before you start. I would hate to see you make 8 connecting rods only to find out they are wrong.
You are not only talking about building a very complex engine but designing it as well. you may just need to become a cad expert.
so I suggest to take a simple plan set like an oscillator and redraw in cad then build it as a start.
The late great Elmer Versburg suggested every part be redrawn before machining. redrawing helps you get the part in your head and reduces mistakes.
Pit falls?? to many to mention. but the best way to avoid them is learn the basics. And above all be patent. you will make lots of practice parts along the way. Even guys with experience sometimes have to make a part two or three times to get a good one or one they are happy with. I tell folks the home machine shop is akin to a College Engineering lab. No mistakes but lots of learning experiences.
If you want to do this READ. First of all get a hold of a copy of Army TC 9-524 available various places on the net as a PDF , get a copy of the Machinery's handbook and a good used older addition of a high school or college machining text book. And then get a book on building the real top fuel engines so you understand the full scale ones.
The journey of a thousand miles begins with a single step. and there will be many steps in your journey.
A good first step? Buy a good quality Micrometer That reads in in .0001" increments and learn to use it consistently. You will never be able to machine accurately unless you can measure accurately.
And folks here will be here to help and encourage.
Tin