I would also ask that anyone responding respects any copyright of the original drawings and does not make and distribute copies unless they have the rights.

That answer if for the full size plans Michael, not the True to Scale ones the OP is seeking.

If the copyright holder is known then it should be possible to obtain a copy from them. If not as seems to be the case here and you can find someone with a set to reproduce then you are either into Bono Vacantia or Orphan Works territory depending on how the company became defunct. If only for discreet personal use then I doubt anyone would bother but if you are going to show them off then a bit of admin is required, the processes are described on gov.uk

I would have thought TTS did not need any permission as their drawings would not be the same design and we don’t know what they used to produce their drawings, could have been a man with a tape measure, some scaling of works drawings or just doing it like I do and working from photos.

What would be under copyright are the 3.5″ design and drawings TTS produced and that is what the OP wants.

Is it most likely that the drawings were originally produced in America? have any been built? Perhaps asking on the HSM forum would yield more results.
Also nowadays there are probably some websites, forums, facebooks, etc dedicated to this loco.