1981 Mercedes Benz 200e Engine problem

Gray market car with carburetor. Engine idles OK, spark is fine as is vacuum. Compression on all four cylinder is 170. However, when revved, there is a lot of rattling noises as if the timing is not correct and the engine has not power. Tried to adjust the timing, but even with the distributor turned fully clockwise, the advance is not enough. Took the valve cover off to check the timing belt, and found a large piece of plastic broken off that looks like a part of a timing belt guide. Timing belt seems tight. Valves have the right clearance. Thought that maybe the broken guide had caused the timing belt to skip. However, everything seems to line up when the crankshaft is set at TDC, the mark on the cam shaft lined up with crankcase, and the distributor rotor points to the notch for cylinder 1. I am not sure what to check next to find out what is wrong.