Smokebox, splashers, cab and cab footplate area would need to be replaced completely, and if the tender is hard-wired in, strip out the wiring in the tender and put it into a six-wheeled version. LMS-type tender would need modification itself. New dome if you're picky.
I've often thought it 'looks' closer to the 800s with the huge boiler, compared to the Royal Scots, but it seems like it'd be an even bigger job converting it