1991 Ford Escort No power in engine

My '91 Ford Escort with 1.9 L 4 cyl engine has no power. It will not rev out when cold, and when warmed up is not much better. There is no kickdown in the automatic transmission when cold, and is slow to accelerate when warm. I could not get the car above 40 mph recently heading out on the highway, until about 10 miles down the road. I have replaced the fuel filter, air filter, air box breather filter, PCV shakes ok, replaced spark plugs (looked clean), spark plug wires. Pulled the valve cover off and ran and saw all valves were opening. There is excessive oil being blown into the air box. Had an engine diagnostic done which suggested internal problems, but inconclusive. Could this be a timing belt off one or more teeth? Timing is not adjustable I noted. ANY possible answers would be much appreciated. Thanks, Clint