If there is oil weeping on the head level ( a few cooling fins below the outer spark plugs #1 and 4) and your compression looks good, There are O rings on the head gasket level that seal an oil journal on each side that wear out (loose the seal). that will leave oil all over where the jugs meet the head. It's a common issue. If that is the problem, the top end will have to be removed to replace that and the head gasket.
I usually get a Vesurah gasket set, covers almost everything .does not come w ith replacement pucks that seal valve train from headbolts so take them out carefully so you can reuse them (or Honda still sells them).
If that's as far as you need to go , the top end can be removed in-frame, no trouble,but if you need to repaint the frame and engine, removing the engine is no biggie either.
If you have multible leaks , the Veserah kit comes with side case cover gaskets but no kick-start or shifting rod seals.it's probably time for those too if you haven't changed them.The kit does come with valve seals ,if compression check was off, re seating the valves would be a good thing to do while your there anyways.
If you plan on splitting the case for some reason the crankshaft gaskets are n a for the 550 but the 650s used the same ones and was still available last summer.
If you know you have had a bore job on this engine and you have good compression treat those rings like gold, stock size is still avail. any oversized rings are not .
Sohc4 , down load a manual and read before putting a wrench to the motor...(also been on that site for years) good, knowledgeable,helpful folk there.
If you haven't taken down an engine yet , it's not hard, just have to pay attention to the manual. Will you be posting the repair here and there?