Hi Ian, Actually NG rolling stock was anywhere from a foot to a foot and a half narrower than standard gauge. The addition of standard gauge trucks and wheelsets wouldn't make the conversion as in a lot of instances the sideframes would be sticking out from under the edge of the carbody.
The conversion of locos is even harder......the frame would have to be widened to accomodate standard gauge drivers and etc.
I've converted some standard HO cars to HON3 by shortening the width and adding NG trucks and the result was good. These were some Roundhouse Overton Passenger Cars and some Tichey Group ore cars.
You might want to take a look at the MDC/Roundhouse Line of locos. Most of their narrow gauges locos are just generic versions of some of their standard gauge locos and they look just the same.