Since this engine has not been sitting very long try the following. If the engine has been sitting for years - don't try this.
First find out if you have a stuck valve or valves and which cylinder(s) - finger in the spark plug hole.
The edge of one valve can be seen through the spark plug hole. If valve is stuck open - using a soft metal rod - tap (rap) gently (not hard enough to damage the valve) on the top of the valve.
Other valves. Remove the manifold and valve cover. Spray the offending valves with solvent. Spray in the port and in the valve spring area. Wait, spray, wait, spray. I'd give it at least 24 hours.
Still not free. Take a feeler guage or thin brass stock. Rotate the engine backward until you can insert the stock between the valve lifter and the valve stem. Turn the engine by hand or hand crank as though you are starting the engine. You will feel the resistance when the gap between the lifter/stock/and valve stem is taken up. With the hand crank put a bit more pressure on the valve stem. Rotate the engine backwards then forward.
Usually, once the valve begins to move - it will free up quickly.
You probably should remove the sparkplugs and add a tablespoon of oil to each cylinder before doing this task. This will make rotating the engine easier.