The first order of business would be to pull all the plugs and perform a compression check. Be sure the carburetor is opened up while doing this. A compression test will at least give you an indication of any potential engine problems. A leakdown test while the spark plugs are out will further isolate if specific problems are in the rings or at the valves. Assuming the engine checks out okay, then a complete tuneup would be recommended which would include checking the valve lash.