Just cut it off flush . Cut away the sheeting top and bottom, front and behind, near the spar.
Cut the slots into the center ribs for the jointer.
Glue different joints to the front and back of the spar. Use clothe line clips to clamp the jointers to the spar.
Sand the new jointers flash

Alter the diehedral if you want to. Just use tri stock (center ribs to jointers ) after re angling the center ribs accordingly
Then just resheet.

I'd cut all the sheeting and re-sheet like Campgem said becuase I hate sanding fiber glass. Plus I think it's just easier and
the final finish will also look much better. Plus I'd add another jointer to the rear spar just for kicks.

Cut the center Alignment tab. Install a center piece dowl.