Low flow toilets of that generation were almost 100% junk and would hardly be worth repairing. A $90 toilet? Absolutely will be a build grade and be nothing but trouble. Maybe 30 years ago a decent toilet could be found for that kind of money, but not today. If you shop around,you could likely find a Toto Drake for somewhere around $250, depending on the seat chosen. Anything less than that I would not put my name and reputation on. If you and the owner are committed to going cheap, then you can find all sorts of builder grade toilets for $100, but you both need to understand these do tend to clog frequently and have poor bowl wash.