Another option would be to take the mechanical part of Anthony Mount's Garratt semi-portable and drop that onto a boiler and wheels which is basically what Garrett's did though I think they started with a portable as can be seen on Graces. Drawings were in ME and it suits a 4.5" dia boiler as is but could be adjusted to what size tube is available in the scale you want.
Main thing with a portable is that the backhead, wrapper and throat plate are all on view so you will need to decide how to deal with the rivits – rivited boiler – dummy rivits in copper boiler – or clad the firebox with a fake but detailed exterior which l;eaves you free to do the boiler as you want.