I would imagine the roller rockers are creatively specced from some more mainstream domestic engine with a shaft rocker system, but I wouldn't know the application. I think John Parker and KgTrimning sell sets. It's a fairly subtle tweak, however. It doesn't really change the nature of how your cam works or the flow characteristics of your intake/head/exhaust. It just adds a little more power. All in all in terms of bang for the buck they should probably be fairly far down the list of tweaks. Since the head is already off you'd get much more improvement by putting in a different cam.

I'll let the experts advise you on it, but the weak point on a Volvo head is the constricted, tightly bent exhaust port. It won't make much difference what else you do if you don't improve that.

Depending on what head you have (and if it has already been shaved) carving it down some to raise the compression ratio can help make the engine a bit perkier. Too high, of course, and you'll need to run premium or retard the timing so much it actually makes less power.

Of course, improved flow in the head isn't of much use without improved exhaust flow. The double downpipe exhaust manifolds are pretty good, but definately ditch any single downpipe old style manifolds. OEM sized exhaust systems can be restrictive as well - 2, 2 1/4, or even 2 1/2 inch exhausts will let your typical steet 2 liter engine breathe freely.