When you say easy you mean cheap. Replacing valve springs is generally not cheap. You could swap the head and I would suggest it if you've been running around with this problem for a while. I'm not sure about this diagnosis...I think the problem you're describing could also be the lifters (it's probably there at idle you just aren't hearing it). Assuming the fact that you're driving the car at the stock rpm redline neither of these parts should fail at this point unless they were deprived of oil. I would call up the mazda dealership and ask the service manager if there's a recall on your car's engine head.
Replacement used head and installation would be around 600 bucks if you buy the head yourself (recommended head gasket replacement and new timing belt/chain; with the head off it wouldn't be much more). Head rebuild probably around the same because it's fairly time consuming and the parts aren't cheap. There might be more damage to the head than you know that's why I recommend a replacement warrantied used head.
