You really need to get in there and do some diagnosis. From your description, a rebuild is not warranted.

I would start with a manual and a multi-meter. Pull any codes out of the computer and then start checking things like the TPS to make sure they are within range.

I would highly suggest a compression test to see where you really stand mechanically. A leak-down too, if you can do that (or are willing to pay for it).

You very well may be at a point where you have a few things that are not necessarly "bad" but together they add up to your problem. A slow O2 sensor, a exhaust system that needs attention, some bad plug wires combined with a weak coil and anything else, may be enough to allow the MPG to drop like you have described. But, you won't know until you do some of the leg work.

Good Luck,
Michael