So an update, over the past couple of months, I've slowly sanded off the original varnish and just about finished recoating.
I used 3x coats of water based wood stain (Dark Walnut), then 3x coats of Danish oil. I picked up a repro butt plate and tarnished with some salt & vinegar, it's not perfect fit, but better than the metal one it comes with.
Happy enough with it, it's far from perfect, but then that adds a bit of character 🙂
Original vs real steel
Sanded
Reoiled
Butt plate