Hmmmm.
I've actually had good luck tapping a 4.5mm bolt hole (old Shimano style) to 5mm X 8 using either a 'poor man's tap' - essentially a hardened 5mm bolt with a pair of vertical valleys Dremeled into it- or a 5mm bottoming tap so that it would accept a now-standard 5mm mounting bolt for a cable stop or downtube shifter.

Just be careful and take your time. No need to drill anything out, just re-tap the threads.