The 2.0L engine in your Contour/Mystique/Cougar utilizes a timing belt to drive the camshaft from the crankshaft's turning motion, and to maintain proper valve timing. Some manufacturers recommend periodic timing belt replacement to assure optimum engine performance, and to make sure the motorist is never stranded should the belt break (as the engine will stop instantly). On some vehicles (those with "interference" engines), this is especially important to prevent the possibility of severe internal engine damage, should the belt break.
Whether or not you do decide to replace the timing belt, you would be wise to check it periodically to make sure it has not become damaged or worn. Generally speaking, a severely worn belt may cause engine performance to drop dramatically, but a damaged belt (which could give out suddenly) may not give as much warning. In general, any time the engine timing cover(s) is (are) removed, you should inspect the belt for premature parting, severe cracks or missing teeth. Also, an access plug may be provided in the upper portion of the timing cover, so that camshaft timing can be checked without cover removal. If timing is found to be off, cover removal and further belt inspection or replacement is necessary. Service and inspection procedures for the timing belt can be found in Engine & Engine Overhaul .