aluminum rib, aluminum spar, holes drilled but screws broken off

I'm rebuilding a Super Cub wing and the flanges on the ribs and the
spar caps have been drilled perfectly so the #4 steel screws go right
in, that is, if a screw has not been broken off. In which case I have
the option of redrilling and turning the spars into Swiss cheese and
losing ac43-13 edge distance on the rib flanges, or drilling out the
broken, embedded screws. Trouble is that when I try to drill the damn
broken screws out they harden and the drill bit becomes useless.

Looks like I need a #45 drill bit made of diamond or carbide. Ideas?
Thanks.