Dave,
Before attempting to have Helicoils fitted, make sure there is enough 'meat' around each lug - otherwise you may end up bursting the side out of the lugs. I think a better solution would be to open out the holes to 6mm. This is the next larger size that is commonly available and the pitch (1mm) will be slightly coarser (much better in cast alloy). Henry and myself resort to this on our sprint engines without any problems. The only drawback is that you will need to get special screws made if you want them to look original - but in the meantime, ordinary hex heads with get you mobile again.
Regards,
Eddie.