I thought I might have been in a similar situation when mine died. I decided on the rebuild route becuase I now have a known quantity as opposed to a secondhand unknown quantity of unknown history for the same cost that may or may not go bang.....
Mallory Pistons as you say - cheap as chips. I assume yours is a standard merc +0.015" +0.030" bores for the standard pistons. I found somewhere that would do a +0.045" for Mercs, at not much more than the standard oversise price.
If your man can't get his cutting device far enough in*, there is a place up here does have one that will reach. Did all three of mine for £200 odd, including skimming the back face to make sure it was all perpendicular. Will need to look the number out, so let me know if you want to go this route & are stuck.
Disconnecting the autolube: Remove the pump, plug the oil inlet holes, then divide by 50 when you fill the tank! It'll smoke a bit more at idle, but that's about the down side of it unless you do a lot of pottering at low speeds.
* a vague attempt to prevent it all going bilge-ward