Me and my Crew have an old national 12 (1962) and are restoring her to sail in regattas for fun, not serious National Championships so.... we wondered is it ok to rig the boat to sail with a spinnaker just for fun in a 'mixed centreboard' class?
First. You will be out of class so no PY so no race.
Second. The extra power might pull an old boat apart.
Third. Is the cost of fitting the spinnaker. You will be rigging a 1off system, and you might need a stronger mast or fit a puller. The puller would be £70 plus. (Just fitted a new one to my Merlin).
I rememeber an old friend fitting a trapeze to his old clinket boat once, "just for fun". cerainly made it go up wind quicker when they could get out on the wire. didn't break the mast either.
This has been done before, with an assymetric too. It should hold as long as you dont let off the shrouds, or weaken the mast too much drilling holes in it. The Old Procor C masts were pretty strong and used on several spinnaker boats. With a keel stepped ali mast you wont need a "puller". If there is a tendancy to invert you can always chock the back of the mast. A lark sized kite would do.
Andy