Charles, The dimension for rowlocks aft is detailed in the Wayfarer Book (available on CD).
It is worth getting the book as it has a lot of information in it.
And I suggest you sit on the thwart in a rowing position with oars to check – before drilling big holes!
Start with a small pilot hole to check the position.
I’m afraid “my supplier” was a neighbour long since moved.
I have not looked recently to see what is available off-the shelf. You do need to be careful that whichever rowlocks you use match the fittings. We used fairly standard galvanised rowlocks.
hope that helps