At that displacement it could still be a car engine. That is the only thing that makes sense to me with the R/C carb and rear exhaust. I think I remember having seen adds for OPS car engines back when I worked a lot at the hobby shop. They are a known high performance engine manufacturer. If it was a heli engine it would have a big head and cylinder fins for extra cooling. If it was a boat engine it would have water jacket cooling. At this point, you got me!
Type at you later,
Dan McEntee