Yes... I thought the 900 Monster was steel? Could be wrong but the S4 one is definitely ally.
If you get one, make sure that you get the right bolts to fit it to the swingarm pivot. The steel arms that I presume are on the 600 have a pinch bolt in front of the swingarm pivot to clamp it onto the pivot. The ally arms have a hole behind the swingarm pivot that some kind of pinch bolt goes through - the arm itself simply has a hole there.
I have an S4 ally swingarm from eBay that will fit my ST2 (all the other part numbers for the rear suspension of an S4 are identical) but I didn't get the swingarm pivot bolts with it.
The parts manual doesn't make it very clear from the drawing what type of bolts to use (it can't be just a straightforward bolt, there must be some sort of cotter pin), although the parts manual obviously gives the part number. Trouble is it's impossible to get the part. I've had some on back order with a couple of very reliable suppliers for nearly a year and neither of them have turned up trumps yet.
So, make sure whoever supplies the swingarm supplies the fittings
There are two distinct Monster swingarm types depending on the build date of the bike. I know they changed but as for when...? Try a u2u to CK & AK, Alan will know.
Tonio the difference is easy to spot the earlier monster has the hoop similar to a 851 etc, the later ones have a tie rod similar to a 916 etc and this is the type found on the S4 monster
The S4 swingarm will not fit the M600 as stated above. The suspension changed from the Hoop style to the newer style ST type frame with the 2001 S4 and the rest of the M series bikes in 2002. An older M900 alloy or 851/888 swing arm will work on the M600 though.