order it through a friend at dealer... it will cost you a bit, don't remember but wasn't that high... IIRC something like 1000eur to 1500eur as there are different versions from 95 to 97.. as you're there "waste" other 2000-3000eur for the carbon fiber airbox... ah sorry then you need the ITB... but then there is the 5,5kg flywheel that's appealing and then other 2000euros for the crankshaft, and what about those 316'/300' camshafts for 1700eur? every lifter is 100eur, the oil pump is 2000euro, the last evolution exhaust manifold is 4300eur... for 8000-9000eur you should have a full S42 new exhaust
PS.: I don't know if BMW make those parts anymore and a normal man can't buy them, you just need VIP friends (money naturally:)) in there or search in racing teams that use these cars, they have always spares for sale if you ask them
prices above where from oem bmw motorsport part catalogue

so how much money I wrote there?... mmm maybe with that money I would build an RWD homemade seveneque kitcar (aka locost, caterham lookalike just to give the idea) that weight less than 600kg with the power that I want and LSD and have a lot of fun, with all respect to M42 and S42:D