It's valve train, not valve terrain
The rs valves and stuff should be fine.
The intake manifold shouldn't be too big a deal....depending on whether you run an intercooler or not, you'll have to do some custom fab/piping work anyhow.
For the exhaust, yeah if you keep the stock manifold, you'd definitely need custom piping. But I honestly think that'd be more of a hassle. Just get a WRX header & up-pipe. It'll work.