Hi Eric
Had one go at converting a Tower rtr GW 1361 tank to S7 several years ago. Now runs well and looks OK after I replaced the incorrect smokebox door - BUT the chassis was crap and probably needed replacement for fine scale use! I ended up with new rods (Premier), wheels ( A Harris), motor (Maxon) and frames (Me). All a bit expensive but the loco body was OK for the most part and I would guess came from a different source from the aforementioned 'bottom bits'. I think the J39 is a more recent offering and perhaps things have improved but Tower have given me the impression that they are not S7 fans!
Julian