A mechanic told me to replace these parts for my car:

Long Louis

Hello all,

I drive a 2003 Honda CR-V EX Automatic Transmission with 199,561 miles. I have a permanent P0301 code and was told that I have additional P0302 and P0303 permanent codes. Granted I already replaced the spark plugs and ignition coils, along with a knock sensor that was causing performance issues on my car. I was told to have the serpentine belt, belt tensioner, valve cover gasket replaced along with a valve adjustment. I told the mechanic to not buy a serpentine belt because I already replaced it this year and it doesn't seem to be wearing out. I was surprised to see that he said that I need to replace the valve cover gasket, especially when nothings leaking. He told me that he hears a rattling sound on it (if my memory serves me well) due to the worn out tensioner. I sent the car to be fixed on Thursday, but was told that it would be done on Monday because they are backed up. They work limited hours on Saturday (8 AM - 12 PM) and are closed on Sunday.

I don't want to wait any longer and I just want to have my car fixed, but some of the recommendations like a valve cover gasket replacement and serpentine belt replacement are fishy. Maybe I can understand the belt tensioner, but even I am somewhat doubtful about that and may need to check it myself. I certainly can understand the valve adjustment. I just feel suspicious about the services I'm being charged.

What should I do? Please help.