Things like gaskets dry out when the engine isn't run regularly. The engines will have to be torn down so that every gasket, hose, belt, etc, can be replaced.
Given the way they were stored, I'd be surprised if the internals weren't rusted. Pistons, rings and cylinders, for example.
take all the plugs out, fill plug holes with Marvel Mystery Oil. manually turn it over using the fly wheel or by disconnecting the start and turning the key. do that once a day for a week. drain oil and fluids. refill with new and replace plugs.