It is pretty easy to pull off the rocker cover and see what it looks like inside there. The cam should be free of galling and scoring on the bearing surfaces and lobes. Put some engine rebuild lube on these spots. Clean out the oil holding wells and fill with clean oil.
If this area looks good, probably the rest of the engine is OK.
If you feel really handy, you can replace the exhaust valveguide seals using compressed airtro hold the valves up. The seals will be brittle and leaky by now.
While you're in the top end you can check for cam chain tensioner operation, loosen the locking screw and the rear tensioner leaf can be pressed down to work the action if you remove the little cap thing that holds it in place.
You can look in the 1 and 4 sparkplug holes with a flashlight and see how the bores look, they should be relatively smooth and free of pits and scoring.
If it looks pretty good, start the engine before deciding on further action. loosen all the tappets all the way and make sure they are not jammed against the valvestems while reinstalling the rocker cover. Set the lash to spec. Be VERY careful with the rocker cover bolts, the head metal is pretty soft and it is extremely easy to strip threads out.
The main bearings are good for a long time if the oil is changed regularly. I think most shell sizes are still available from NOS deaalers.
Occasionally a seal and gasket set shows up on ebay, possibly David Silver or CMSL might have them. The seals are mostly standard sizes and can be bought from industrial suppliers, but a kit makes life easier and I think the main drive shaft seal is a special item.
I've come to believe that replacing rings on a small aircooled engine is not worthwhile. If it has a problem leading you to want to change rings, you should bite the bullet and put in overbore pistons.
Top end gasket sets are readily available, Athena has a good set and you will probably find an original set at a Honda or NOS dealer.
