Plymouth Neon, trouble accelerating?

I have a 2000 neon that I just bought, and I've already replaced both motor mounts and the exhaust manifold since it was cracked. The car accels better since I replaced the manifold, but it still has a lot of trouble getting up to 40. Now, could this be another problem, or is this simply because of the 4 cylinder engine? I've read that neons are "known for their power" but my poor little car has no power :( When I was checking out my spark plugs, I found oil in the first plug that's under the spot where you poor in oil. I marked it off as the owner before me being an idiot, but my uncle said that it could be a valve problem. Does anyone know or have any suggestions?