In peaky, small-displacement engines(like that of, say, my 2004 Honda S2000), horsepower is good, but torque is scarce.  A stroker kit helps, but longer stroke can cause problems at high rpm.  So what is the weak link in the chain?  I should probably get stronger valve springs and stronger and lighter valves and retainers, but what about the rotating assembly?  Lighter pistons could help, I'm sure but should I get lighter rods, like titanium?  I can bump up the displacement from 2.2 to 2.5 liters.  This will give me a torque boost, but will my upper rpm range be in danger?  I don't want to beef up my block only to grenade it later when I add a blower or turbo because it revs too high.  Any insight is appreciated.  

any good S2000 stroker kit should tell you what how it will affect eventual figures.  

Generally, though, I think the S2000 is fine with the stroker kit (and under boost).  The problem with stroke/bore ratios and high rpm only really starts in the KA24 from Nissan.  It actually cavitates the oil and does very poorly higher up because of the engine layout.  

if you want more lower end torque what yu will basically have to do is change the piston speed, what you will have to do is have the piston move really fast from TDC and slow down as it gets to BDC, that way you will get more low rpm torque, now the only way to change that is to change the rod ratio.  

rod ratio = Bore/Stroke ratio.