I've had so much time in the saddle with these bikes, and have researched and tested first hand what does and doesn't work for them. Some pointers and advice worth summing up and giving out...
What was once classed as a downhill travel bike is now really modern xc, so ride it as such! I used mine as a fast xc/trail bike.
Get the best front shocks you can, and stick within the 120mm Rockshox (SID or Reba), 130mm DT/Pace (XMC130 or RC41), or up to 140mm for Fox as they've usually a shorter axle to crown race length.
A quality air rear shock will transform the rear. I've run DT (firmer and less reactive on gravel and light bumps) and Rockshox (the Monarch has been fantastic) on all mine, but I know many others have had equal success with Fox. One trick that my cousin has done was to run the 200/57 rear shock on his as 200/50. This really stiffens up the rear end stroke and prevents some of the wallowing that the longer stroke shock can be prone to. It effectively looses an inch of travel at the rear, but can be more balanced for it.
Don't faff about with old disc brakes - use modern Shimano Deore or better.
Go tubeless tyres, and run 2.35/2.4 tyres at 20 to 27psi dependent on your weight and conditions.
Fix the cockpit - use 680mm bars or wider, and team it up with a 70mm Max stem. The steering becomes so much more direct, but too short on the stem and you'll start to off-balance your neutral centre of gravity.
These are what I'd call the 'must do's'. Things like gearing and pedals are up to you and won't make much difference for how the bike feels. I chose simpler gearing systems each time I would build one. The last one would have been 1x12 using Sunrace shifter, Sunrace 11/50 cassette, XT 11 speed rear mech. 11/12 speed Shimano works on the same actuation rate so they're compatible, but Shimano won't tell you that directly. I'd use a 30 or 32 ring up front, and that'll get you up and over everything.
