I did say he could fix and use his own toilet - any non-conforming used one must have been removed from your domicile, not something you picked up off the street used (it wouldn't be being installed in your domicile after removal)...but, the only way to use some other, non-conforming toilet is to physically import it into the state for use in your own home. Any other, non-conforming toilet that you might install must either be new from stock somewhere already in the state, and the dealer cannot order one since it won't be installed in his home when you're buying it. Try to find a new, non-conforming toilet to import...it's going to be tough. Try to get a building inspector to approve installing an old one you picked up off the street. If someone sends you one as a gift, you're not the importer and unlikely that it would be being installed in the same domicile, which precludes it. If any dealer has stock new from 1992 (when the federal low-flow, water sense requirements mandated what could be manufactured), I'd wonder how they stay in business.
The intent of the regulation is to allow you to use what you already have, but any new fixture to meet the current requirements. The dealers cannot buy new stock that doesn't, because they can't sell it and the Feds have mandated water saving functions for decades now, and nobody makes them anymore to buy. Canada fairly closely follows the US low-flow requirements, so getting one from there would be iffy, then there's the practical situation of shipping a brittle, heavy, piece of porcelain and having it arrive intact.
So, to dumb it down...you can reuse an old, non-conforming fixture IF you reinstall it in the same domicile (can't do that in public buildings or commercial - those aren't domiciles), and you can't buy a non-conforming one locally because they can't buy them or import them or sell them. That leaves you trying to find one somewhere that you can import for yourself. For practical purposes...when the old one is no longer viable, buy a new, conforming one.
Back to the OP's problem...it is probably gunked up with mineral deposits (generally, requires an acid wash...dangerous if you don't know what you're doing (noxious fumes, potential chemical burns, may not work). If you can remove them, it may restore its functionality and could last decades. Given the drought in a good portion of Texas, there is incentive to replace it with something that can save some water and, many of the new ones will perform better than some of the old ones.