On the px I would use the one with the gold insert....all of the J wheels should interchange....it has been my experience that the plastic used on most of the simplex stuff will eventually self destruct so the all metal versions of the derailuers are the best in my opinion.
I have replace the JW on my SRD with Shimano...you will have to do a little drilling for the bearing but thats no big deal. You just have to find JW that have enough meat on them in the center.
Good Luck Ben