I have corolla 2010. it was running awesum till last month and was giving me prety good mileage on cng like 100/110 on full tang. but it has suddenly started to gime 80/85 on full tang and has goten a bit hard. I got the airfilter changed, spark plugs replaced, tunning done. the drive is okay now but the mileage is still the same. one of the expert cng mechanic said to me that it needs tappet adjusment and cleaning. i have few questions

1. is it a hard job to do like should i search for an expert mechanic as i think there is gona be opening of the engine head.
2. how important is it to get dis work done? bcoz i always have had bad experiences wid mechanics. i hated in ma previous cars when the mechanic has opened engine head to replace gaskit and stuff and ultimatly the engine sound and everything is screwed.
3. is there any reliable place where i can get this tappet work done?