Maybe, and yes, thanks for the suggestions. However, unfortunately I think that the extra effort and double sourcing is likely to put off the causal kit builder, or certainly the beginner. If you have to scratch-make parts, like the roof, and purchase some bits separately, it's no longer simply a complete kit ready to assemble. It's basically just a scratch-build aid and probably a little disingenuous to call them kits.
Although I can make parts from plasticard, I am supportive of developing kits to be complete packages so that they attract enthusiasts in to having a go at some constructive modelling. The NGS has certainly been working to flesh out kit contents by including wheels, bogies and couplers (missing from some ranges they have acquired) as well as producing appropriate decal sheets.
The development of kits over the years has seen to move towards supplying all the bits and parts necessary to make a complete running model. It seems like a backward step to a time when many kits didn't include wheels, couplers, decals, etc.
However, I understand that the Farish chassis is no longer available as a spare part from Bachmann and the roof part is a casualty of the RTR model being discontinued (with rumour that the tooling is worn out).
Oh well.