I would get a piece of plywood, 3/32 and cut a spar doubler with the same taper as the spar, but trimmed off so it is 1/16 under the skin line on top. Then take an .06 carbon rod and use that as a spar cap reinforcement.

You get it into the wing by slicing thru the skin right next to the existing spar with a dremel wheel. The slice needs to be as long and thick as the ply reinforcement. You can remove foam down to the other wing skin with a soldering gun or hot wire used with just enough heat to melt the foam away.

You can put the ply in with epoxy or polyurethane glue and you can put the carbon rod over the top by sliding one end in under the skin, then the other. Make sure it all has a good coat of glue. Fill the skin gap with balsa. Sand smooth and cover the whole crack with 1 oz glass. Feather-sand into the rest of the skin, prime & paint. You always want to cover cracks in skin glass from the outside with more glass, even with light glass. If you just glue/paint over a crack, it will reappear.