The only shops I can speak of with a history of wokr on the KA24's is Rebello and Sunbelt. Sunbelt did the head in the KA that I used to race. The head is where the power is, so don't take any shortcuts there. Even for a refresh, I would change the valve springs, or at least the exhaust springs. (retainers and locks also) They take a real beating. It'll likely need rod bearings also. I've used both the Nissan rings, and the gapless style in the Nissan .040 over pistons. The size is common enough that you can find some performance ones if you desire.

As for when to do it, you're own your own there. Some will say if it ain't broke, don't fix it. Some will say that an ounce of prevention...... You get the idea. Ask yourself some questions that might point to refresh time. How's the oil pressure? Is it the same as when the engine was fresh? How's the power? Dyno sheets will help tell if the HP is falling off. Oil temps still the same? An increase in oil temp might indicate a bearing getting tight-or other clearance issues.