Hey, my dad picked up some oversize pins for my car, as the doors look a bit sad.
Is there a particular way to go about fitting them? Should they be professionally
machined to the correct size, or will a drill do an accurate enough job?

Are they really oversize pins, or standard pins with bronze bushes? (this is the usual method used).

The pins and bushes are easy to fit, OFF the car. Don't try it with them attached...
The pins are punched out upwards, I sit the hinge on a block of hardwood with a groove and hole in it. The new pins are fitted from the top. Top edge of pin is knurled.
The standard pin is 0.250" dia, the bush is 0.250" inside and 0.3125" outside.
I drill them (rear half of hinge only) in a bench drill- first to 7.5mm, then use a 5/16" drill with the corners stoned off, so it hardly cuts (old toolmaking trick). It will then give a neat size hole 0.250".
If the holes are worn bigger than 5/16", I drill out to 8.5mm, then 9.0mm (with a drill stoned as above), and make new bronze bushes 9.1mm dia.
I then press the bush in using the bench vyce.

Pins and bushes are available from Karcraft etc, but I got mine in from MiniSpares UK years ago.
No, sorry- before you all ask, I'm not really looking to do hinge work for people..

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.