Be carefull with that new spring. The spring doesnt need to stiff, just enough to excert enough presssure to keep the wheels against the rails, too stiff it will push the engine up off the track.
I have a Barry's pilot mod for one of my engines, I havent added it yet.
I used self adhesive lead weights on another one, but I have one unaltered later Bug Mauler and its been running on my indoor layout as is, stock, without any trouble, backwards and forwards, over LGB switches and up and down my 4% grades.
Are you using Aristo switches? I had nothing but trouble with the pilot wheels derailing on Aristo frogs, so much that I swapped out what Aristos I had for LGBs and havent had a problem since.