Dave,
I am not familiar enough with the way Lionel has the cylinders cast. One would think that it shouldn't be such a hard thing to do in order to install the multiple bearing crosshead.
One other thing that I haven't looked at, because I don't need another J, is the tender. There is a difference in that engines 600 - 610 have a roll-top tender deck and 611 - 613 have a flat top tender deck. Lionel did get this right with the original scale J 605 and 611. I like my 605 with the roll-top.
MTH did make the flat-top tender, but, I have no idea if they ever made a roll-top for the earlier versions.