I never had a problem with the gas gauge. The specs are the same, so unless you get a bad gauge you're fine. The gauges are not very well made, so of course you could get a bad one, especially if yours is salvaged from a wreck. The one I got for my 95 had a bad voltmeter, because its innards are glued together and sometimes come apart. The gauge is calibrated by adjusting the position of the glued-on piece. I can't remember the exact construction details after a few years, but it is possible to open the meter up, and reglue it so that it reads correctly.
I don't know about legality on a reset odo. I didn't worry about mine, since I intend to drive it until it is junk and mileage is not an issue, but figure it should be all right as long as you're honest when you sell it, and explain what you've done. I was able to reset mine with a very fine dental pick, reaching in between the barrels to move the individual barrels. Work from small to large, so that when you move the numbers, you don't roll over a larger one that you've just reset, or you'll have to do it all over again. It's been a long time since I did this, so I don't remember exactly how I did it, but it wasn't very hard.