Is it worth replacing timing belt and valves?

My car, a 2000 saturn lw2, has been rough, with a knocking like sound when running. It went onto the shop where they replace the mass airflow sensor, but it did not fix the problem. The car did seem to run better until one day as I drove it it started sounding really bad, and loss of compression? I got to the store I was going to and turned off the car. I started it up about 3 times letting the engine crank up and turn it off. After the 3rd time, I went I to the store. I came out and tried starting the car. All it did was whirr, and did not try catching. I was told it was the timing belt. My question is: is it worth it replacing the timing belt and any bent valves.