I have an Avid with a Subaru engine in it an this is what I do for an annual to the motor. Change oil and filter (cut open filter to inspect for metal fragments if you wish) ,.Check and adjust as required tappet clearance ( the clearance will depend on cam used ) flush and change coolant ,check cooling system hoses check carburettor security and if you have bing carbs and the diaphragm has not been changed replace the diaphragms , syc carburettors (if you have 2) ,check engine mounts and mount frame for cracks ,check exhaust system,check /replace spark plugs as required ,inspect air filter . What I find works for me is after I have done all this and before I put cowls on I sit down have a cup of tea a deep breath and look again at all I have done just in case I for got something.