Dad slotted his with a die grinder, and once they were assembled again, ran a grub screw between the flanges to hold it in position. As long as you mark it out properly a die grinder won't be an issue.
Failing that I think the cheapest option would be to take them to a machining shop. Would require an indexing head and a mill, both tools a machinist shop will have, and should take about an hour to do the pair. Hell, I'd do them for ya at work for nothing but our indexing head is only a baby one - swivel hub won't fit.
Ben

