Hi, Suncake-

An LJ or SX series Simplex should still fit. I just measured the bolt diameter on an LJ and it is slightly smaller than the bolt diameter of the stock Simplex Criterium. Nonetheless, it should fit in the back of the Simplex dropout even given the Dremelling and threading to accomodate the Campy derailleur. Simplexes all have a stamped steel tab to act as a range of motion limiter and it is fixed to the derailleur body so the cut to the dropout will not affect it. Have you considered a Criterium? It's a straight fit to the Simple dropout and despite the scorn heaped on plastic Simplexes, the metal reinforcements make them quite effective. I have been using one on a PX 10 on an old Maillard freewheel and it shifts quite well. Just a possibility to consider and much cheaper than than trying to find a reasonably priced LJ.