Here's an overview of the original Finney inside motion parts - some of which will find their way into no.171:
The LSWR "T9" has 4-bar slidebars, matching cylinder glands and ends on the fret. The motion bracket also looks as if it could be adapted:
...but the real beauty is the pair of cast brass cruciform crossheads which lend themselves almost perfectly to 171's insides:
Here's the connecting and eccentric rods fret from the GW 2721:
....the connecting rods, at just short of 26mm centres, seem almost exactly right for 171, though a bit of lengthening wouldn't come amiss.
I was also sifting through the small pile of Brassmasters LMS loco kits that I have picked up secondhand over the years (never buy them new if ye want to save cash), and realised that the 4F also has working inside motion included in it, which almost immediately led me to consider if the motion bracket, rockers and reversing gear / counterweights could be adapted into the "S".
I think I did hint that this wasn't going to be a quick build, but I'm so grateful to you for providing all those invaluable close-ups of 171's inside motion that most of us would never normally give a thought about.