It can be done, Tim Ellis as has demonstrated on the NGRM Online Forum with his model of 'Count Louis', based upon a Minitrix chassis. Seeing Tim's model made me even more determined to have a go at this myself and a little asking in the right places produced a Minitrix 'Britannia' chassis in a state of flux, it had been subjected to a half-started attempt to build a Barlow stream-outline loco by Andrew Blackwell (who went on to use a different chassis for his project).
After a thorough examination of the parts to hand, I set to work, reducing the chassis from a Pacific to Atlantic be removing the front driving axle position, but retaining some metalwork above to hold various new parts such as the cylinders. A new front bogie came from an N-Brass kit fitted with Parkside wheelsets and various other modifications were made such as reducing the length of the con rods by 3mm. After an attempt to use some old crossheads from the spares box I eventually made new ones from Nickel Silver scrap and brass rod, these slide in N-Brass GCR 'ROD' slide bars, sitting in temporary cylinders for the time being.
The rear truck is a Farish 'V2' spare that Andrew had fitted as part of his Barlow build, it certainly looks the part here. There is one slight problem with the chassis as built, and that is my fault due to my scratchbuilt crossheads... it is a little too wide.
Although the whole thing it too big anyway (think of it as a 7.5 mm scale loco), the extra width could cause a few issues as and when a body is built, although I may be able to loose some of the excess in due course with some cunning new cylinder covers. As to what the cylinders look like will depend on what the final outline is, there is an equal chance of Basset-Lowke Class 30, 'Sutton' outline or a slimmer chance of a Barnes style loco from Rhyl (with a new rear axle of course).
Watch this space....