That looks structural to me. I just removed my keel and the joint is about 6″ below your cracking. That appears to be cracked right at the hull where it is a solid glass protrusion that is actually the bilge. I attached a photo of what it looks like without the keel.
From the center of the pin at the mast head to the bottom of the wire backstay turnbuckle (red arrow)its 356 1/2″ Other photo is the block at the end of the turnbuckle. Hardly makes sense that there is a turnbuckle at the end with a cascading backstay. Not sure what the original configuration looked like. Anyway I believe it’s the original wire and that’s what is there.
Welcome, congrats! I am also a new(ish) owner of #81 from Oregon.I know the original owner of your boat. It was formerly Hellion, out of Grosse Ile MI. He bought the boat new and the brown Tecumseh thingy was a kit developed shortly after the introduction of the Olson. He said it was a motor, some parts and a plan. He and some others did the install. He often says to me “I should never have sold that boat. Fastest I’ve ever gone on a sailboat” He traded the boat in in Ohio on a new Soverel 39. I informed him of your purchase. He was glad to hear she’ll be back racing again. It’s in overall very good shape. Good find, Congrats, and keep posting.