If I find something obvious like a bad rod, piston, etc., couldn't that get replaced without having to rebuild the entire engine?

No.

If you find a "bad rod", you are really talking about a bad connecting rod bearing. This will inevitably have damaged the crankshaft. To repair the crankshaft it must be removed and taken to a machine shop for re-surfacing. Removing the crankshaft requires complete disassembly of the engine. (OK, technically you could remove the crankshaft by leaving the upper parts of the connecting rods and the pistons in place (with the engine out of the car, of course) but no qualified mechanic would do so except under the most unusual circumstances.)

The only way you would find a "bad piston" is by removing the cylinder heads and the oil pan, and removing each piston/connecting rod assembly. If the piston were bad enough to be knocking, it will have inevitably have damaged the cylinder wall. Repairing this would require complete disassembly of the engine and taking the engine block to a machine shop for re-boring. This will also require all new (oversize) pistons.

The only way you would ever replace a single piston or connecting rod bearing is if you planned on reassembling the engine, filling it full of high viscosity oil, and then immediately selling it somebody who lived at least three states away from you.

There is no mechanical failure requiring removal of the oil pan that does not also require complete disassembly of the engine. If you are going to go to that much time and effort, it would be foolish not to do a complete overhaul of the engine at the same time.

I don't know how to say this without sounding like I am a snob talking down to you... but if you don't know this kind of information, you should probably not be thinking about taking your engine apart without an experienced mechanic at your side to offer guidance.

tanstaafl.