Engine Mechanical problem
1999 Toyota Camry 4 cyl Front Wheel Drive Automatic 160000 miles

I have a 99 camry with 160,000 miles. I was told that I should do my timing belt job soon. Can you please tell me which parts have to be changed when doing this job. Is it necessary to change the oil pump seals and the crankshaft and camshaft seals while doing this job even if they are not leaking? Also, I recently found out that the engine head has a small leak and that the gasket needs to be replaced. It is a minor leak right now, but could it get worse later? Should I get this job done now or leave it until it gets worse? If I do end up doing this job, what other parts do I have to change? Is it a good idea to change the head gasket on an engine because I was told by one of my mechanics that after doing a head gasket job, it changes your engine performance and your engine does not perform as it used to. Your engine does not perform as it was designed to by its manufacturer. Is this true? Please help me and let me know what I should do at this point. Thanks.