Pop the valve cover off, shoot a little penetrating oil on each valve stem where it enters the head, and give a sharp rap on the valve end of each rocker arm to see that each valve is free and not stuck. If you find stuck ones, they'll bend pushrods when the engine is cranked or started, so keep working at them until they're free.
Drain the oil and put in 5 quarts of 5w30 and a new filter. Remove all six spark plugs and shoot a few squirts of clean 5w30 into each cylinder, then toss in a battery and see if the engine will crank with the starter. If it will, go ahead and crank it with the plugs out for about 45 seconds, stop and let the starter rest for two or three minutes, crank it with the plugs out for another 45 seconds, put the spark plugs (or new ones) in, remove the air cleaner lid and use the red straw on a can of spray carburetor cleaner to fill up the carburetor via the internal vent pipe, give a shot of carb cleaner to the carb throat to get everything wet, have a fire extinguisher handy, and go ahead and try to start it. If no stuck valves it will probably start and run. You will have carburetor issues and old-fuel-in-the-tank-and-filter issues, and you'll be doing several oil and filter changes in rapid succession.