Please allow me to play 'Devil's Advocate' with you. If someone is worried about availability of inserts, I tell them to spend an extra $85 for a spare set and place them on the shelf in the event there is a "next time". Also understand that we have the ability to have babbitt re-poured into existing insert shells and re-machined. We even have the ability to weld metal into main webs and caps and (re)line-bore a block that will accept Babbitt. We can repair what has been previously modified. The same mindset can be applied to installing O/S stainless seats in a block to go back to 1.500" valves, --OR making 4 new O/S valves.
I totally understand your point and I also realize you are deeply involved in engine rebuilding too however I say all of this to explain it this way, so often hobbyists pass on (mis)information that makes a project seem hopeless for the future. The truth is there are many orphan cars out there being restored where there just is not any replacement parts available. Therefore someone must think the entire process through and fabricate whatever it takes to get the job done. That same mindset should apply to an 'A' engine too. It is my opinion there will always be a way to restore the engine "if" there is a will to do so.