The answer to the original question, is yes.
A X-Mark Pro trigger will fit an old 700 just fine.
However, a stock from a non-X-Mark era gun will require a little extra inletting for the safety to clear in all positions.
And for the piece parts, I found that Midway is plenty resourceful. They had the bolt release, spring, and pins at reasonable prices. Brownells may have them, but for whatever reason of ineptness on my part, I could not find them.