Very nice work from somewhere. If you had more pictures it would help. There are several possibilities and from what I can see it has a pull starter on the rear which makes me think of magneto or cdi ignition. Without seeing the inside I suspect it is a 2 stroke as there are 3 carb/injectors, 2 cylinders for each one and could be port intake and exhaust timed on the cylinder's. This would get it away from having each cylinder sealed in the case. Looks water cooled as there looks like hose fittings on the heads. Not sure if I am even close but as I said I would like to see more or where you got the picture from.
I am an ex HD mech. medically retired and would like to do a project like this. Although I am not cnc but do everything manually but I am learning a lot from being on here. Somebody else may have more idea's on this one. I have converted small air cooled gas(chainsaw/weedies) engines and at this time making a 2 cylinder(60 cc's) horizontally opposed engine for my RC model.