Unfortunately, almost none of the S42 parts would make a practical street M42 engine, except for maybe the internals (minus the 12:1 comp pistons). I'd love to get my hands on a S42 crankshaft.
If you really want to build up a M42, you should start with some 3.2L M3 pistons (86.5mm bore), some custom rods, a full engine balance, and a stand alone engine management system, like Motec, and a mild camshaft. You'd probably have to run a custom front crank pulley/harmonic balancer, since I've heard the stock M42 one fails at 7200rpm. Doing all of this to the M42 would make a fun track engine, but it would not create a torque monster since even a fully worked S42 only produced around 185ft-lbs of torque at a very high ~7.5k rpm.
If you want to be fast on the track, build up a M42.
If you want to have fun on the street, swap in a 6 cylinder.
If you want a compromise between the two, supercharge the M42.
In short, there are no really good bolt torque improvements for the M42, short of a supercharger.