I think anybody makes a replacement metal truck that just screws on and provides exact coupler height. I have replaced trucks but always had to do some work on the car, either cutting the pin off and re-drilling a hole and filling the existing hole with plastic spree or other filler’s, then drilling and tapping the hole. I would have to sometimes shim the truck with washers to raise the car too the right height or on occasion cutting off some of the trucks pin to lower the car to the right height.
You could also consider replacing the wheels with metal wheels on the existing trucks. It lists all the parts needed and the tools used. Some of the companies make replacement wheels, which are excellent.
I measured the wheels on my locomotive and the wheels are 12mm in diameter, which scales out at 42". I suspect you need replacement wheels from Bachmann, rather than a substitute set from another manufacturer, for two reasons. The axles are fairly thick and are held in place by two metal bars. Thinner axles would not be held in place and might need bushing.
The second reason is the axles are insulated in two places, rather than one, which is normal. I suspect that these pilot wheels, pickup currents from rails, rather than one or none. The replacement wheels insulated only on one side wouldn’t provide the pickup; it might short out, without modification of the truck.
The first thing I would check after the gas pressure would be the electrode. If the electrode are deteriorated or gap too much, the spark plugs will not be strong enough and constant. Which allows too much gas to accumulate before it ignites. Causing the "backfire" sound. Make sure the gasket on the electrode plate is good when re-installing the electrode.
Double check that the combustion air intake are clear also. You will find it on the right hand side of the furnace. If the air intake is plugged or blocked the burn will not get enough air, which will cause the same problem.
The gas valve is a spring return and does not draw current when off. Sounds to me like a partial short in the wiring or the thermostat. It might be hard to find. Start by disconnecting wires at various points till the draw disappears.
The steering speed sensor is usually located on the steering rack. Certain GM model trucks and vans have them on top of the column under the dash. It calculates vehicle speed for steering reaction with a variable orifice with pulses from the steering wheel speed sensor. Most are attached with either a screw or tabs molded into the sensor. Usually they have 2 or 3 wires. You need only to disconnect the harness to remove and replace.
Well it is not a bad idea to let the furnace and AC runs for a couple of hours to break it in. The smell that you get is many things; it might be glues and formaldehyde. It is used in many other products so, run your furnace for a bit then run your ac to exchange the air in the unit and you will be just fine.