Any question answered...

Q
what drill bit would be suitabe for drilling a fork crown (501 tubing)?

Do you mean drill or drill bit? You need a pillar drill, ordinary HSS 6mm bit

Is it undrilled? If so, yes- pillar drill.

However if it's an older fork that you wish to use a modern brake caliper on then you can just widen the existing hole with your HSS bit in a hand drill.

Thank you both very much.