Unless the exhaust system pipes the exhaust stream far from the engine, it's still normal to get oil/fuel residue in the engine compartment. The carburetor will "spit" somewhat, there's almost always a small leak from the case/muffler join, and the engine will throw some oil from the front bearing. All of this is common, and normal...especially some oil from the front bearing. It won't get lubed if it doesn't have some oil pass through it. The front bearing is not sealed, but just has a shield against dirt.

You'll just have to clean the engine compartment from time to time. It's part of modeling when you use glow engines.