Are the tappets noisy? if the engine is running quiet and no tappets clacking probably best left alone, its a bit of a fiddly job especially the tappets behind the end of exhaust manifold.
To access the tappets you need to remove the right front wheel and remove the inner wing splash panel to access the engine side plates, you will need new cork gaskets when re fitting the two side plates. The engine should be at running temperature, best to follow the procedure as in the manual, remove the spark plugs too it makes it easier to turn the engine over, its then a standard procedure, work on opposite valves, ie ..if the 3rd valve from the right is fully open check the clearance on the 3rd valve in from the left, and so on, mark them with chalk as you go shows which ones are done