Having worked on quite a few DOHC 420a’s myself, I would recommend not only making sure to have the head sent out to be machined, but also make sure you replace the items in the nearby space…Why? Because you are already there and depending on the mileage on the motor, you may be nearing some other recommended maintenance.
While I was there I would replace:
Timing belt
Tensioner
Water Pump
I would inspect:
Valves and seals
Lifters
(if you are planning on doing more, I’d also check the oil pump pick up tube.)
Just my opinion. Happy wrenching!