Hi, congratulations. When I bought my first one in 1981, alot of people told me about wheels being out of gauge. I remember a couple of the tender wheels were out of gauge but the drivers were and are dead on. As the post above me says, if you have a wide spot in your curves, it will find it . But it is the track, not the engine.
I also own two of the Westside Pacifics, which also had the narrow treads. My second Q4b and both pacifics were dead on gauge wise.
With that said, I have replaced the pilot trucks and tender trucks on some of these engines with slightly wider, more common treads. I should know the width but it is the NMRA standard. It helped with backing up over turnouts or a turn out that might have not snapped completely closed. It isn't a must do but is a fallback if you have problem. The pacifics were more of an issue than the mikes. This was all done when they were running on my previous layout, that had been modified many times and had some iffy trackage.
I also had trouble with the pilot wheels on one hitting the underside of the frame or cylinders on the insulated side and shorting for a split second. I put a delrin wheel set on that one but that was on my old layout that had some tight "iffy" curves. Also, on tight curves backing up on less than perfectly level trackage, you might find the tender front contacts the underside or edge of the cab roof. You can either use the rear hole on the drawbar, insulate the underside of the cap roof or take some other measure.
I DCCed all of my stuff in 2013. Can motor is very low draw and I have a small sound decoder in it....but it has ran flawlessly. I did add an LED headlight. I have ran the heck out of my original and love it. I would pick up another in a minute if I "needed" another mike but I have a total of 6 counting other makes.
As for parts, I can't imagine ever needing a gear box or motor, but there are non westside parts available. There are after marked sources for detail parts too if you wanted to change something. None of my westsides were painted and I had all of them striped down to nothing for painting so am pretty familiar with them .