Sirocco specs: SIROCCO 15 sailboat specifications and details on sailboatdata.com
The S-15 weighs less then 500 #s so you should be able to easily jack it up with a car jack. Getting enough height with proper stability on trailer will be the hard part. Probably the easiest way to check the center board is to launch the boat and dive on it in shallow water. Much easier then jacking it up unless you have access to some jack stands like a boat yard uses. Can you inspect the cable or line that goes to the center board from the cockpit? If it is wire rope and has meat hooks (sharp broken strands) then it is time to replace. If it is rope and is at all frayed then probably good to replace before it breaks.
The S-15 was made around 1970 so a few stress cracks are to be expected after 40+ years. I really wouldn't worry about them unless you notice them starting to get wider after use. Can you check the fiberglass material behind where the cracks are showing? Perhaps with a mirror or two and a flashlight, or a digital camera?
It does not look like you have any stays or shroud chain plates to worry about holding the mast up. The only remaining concern I might have is the mast foot or sleeve that does hold the mast upright. If it is in good shape then just go sailing and enjoy a sloop rigged (boat with 2 sails) boat.