well you can use any 3sge/3sgte camshafts on the 5s, although they both have DOHC, the 3s head was made for two individual cams, each camshaft having a cam gear to attaching it to the crankshaft. but on the 5sfe head yes there are two cams but car "interwoven" ( i guess i can use that word ) with one another, as the intake cam moves it moves the exhaust the same number of degrees ( i know that doesn;t make any sense but i really cant describe it )

See how both camshafts are geared together so the only way to get better camshafts is sending them off to webcamshafts.com and having them re-weld/regrind your cams.
CP makes forged pistons for the 5s/5SGTE (5sblock 3s head)
Crower makes forged i Beam connection rods while Pauter makes forged H beam rods
Proflowdesign.com can overbore your throttle body fore more air flow for $185
there is aftermarket support for the 5s its just extremely costly
The 3sgte valves can be used on our heads which will give you a .5mm oversize or 2jzgte valves giving you 1mm oversize which is going to require head work
TOGA makes HV oil pumps, and main bearings.
3sgte ARP head studs can be used for our head but some stud would be to be grinded down a bit to clear the cams
there is mroe out there but they aren't coming to mind hope this helps