How to replace a pin for a swing keel

[2ndrowe]

My son recently bought a used 14ft skipjack sailboat. He told me that the pin for the swing keel or centerboard was loose. When we got it home the pin was gone.
Looking at the keel, it has a slot cut into the top forward edge with a screw threaded in the end to keep it locked on the pin.
The pin being gone I don't know what size, length or diameter it was.
There doesn't seem to be any way to access the place the pin resides as it is a few inches up from the bottom of the boat up in the groove for the keel.
This leads me to believe the pin has slid back into the hull on one side or the other.
My question is can someone please shed some light on how I can fix the pin or modify the swing keel so it is useable and safe for my son, as of now if the boat goes over the keel will just slide out and into the water and there is no way to adjust when you come into shollow water short of pulling it up and it is heavy, steel or lead I guess.
Any help is appreciated.