The engine sat up for a long time so here is my $.02, take it for what it is worth.

First use a good quality engine flush in the oil following the directions on the container. Then when you change the oil subtitute a bottle of Marvel Mystery oil for one quart of the engine oil. However before you put the MM in the crank case pull the spark plugs and put a teaspoon full in each of the plug holes. Leave the plugs out and crank the engine for about 15 seconds, let the starter rest and crank it again. Then put the plugs back in and start it. It will smoke like the dickens for a few minutes till all the oil burns out of the combustion chamber. Let it idle until it comes up to normal operating temp and then another 15 to 20 minutes to let everything work.

If the rings are stuck from sitting up there is a good chance this will free them up. If it works it will save you a rebuild. If it doesn't your out about $20 for the oil and flush and a little of your time.