the most critical things are the reciever barrel and bolt.
you CAN get them seperate and have a good smith that knows how to barrel a m1a/m14 barrel it and using the bolt you should also have at the time set the headspace. so you need those three things as a minimum before seeking a smith to get em together.
after that its just parts. unlike an ar15 there really isnt a standard anymore for m1a parts. i would strongly recomend sourcing as much as possible from the same source and even then you may be facing fitting work
it can be done
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