First of all, do you have a Sunray or a StarRay? Not that it matters much, but they didn't make the Sunray in '91, only '89-'90.
Now if I recall, the 170 StarRay is basically an extended 1550, with dinette in the front, couch in the rear, and midship bath and kitchen. The roof will be all aluminum, and I'd recommend you don't replace it unless it has multiple holes throughout. The aluminum roofs are some of the best available and will outlast any rubber roof.
Please explain in more detail what the previous owners did wrong. If it was just a problem with sealing the seams, just scrape off the old stuff and start over. Removing the roof requires removing a lot of screws (hundreds) just to expose the edge of the material. I've never done it, but I'd think if you already had the materials, it would take a better part of a weekend to do, if not longer. Remember, all edge seals have to be removed, along with the AC unit (if applicable), roof vent(s), TV antenna (if applicable), and stink pipe caps. Be sure to have a few rolls of putty tape from your dealer on hand for reassembly. You don't want to use the old stuff as it is flat and will probably leak if not put back right in the same spot. If you have an AC unit, pulling the roof will also most likely require a new AC gasket to ensure proper sealing of that. I believe they're about $20 or so at the dealer, depending on your model of AC unit.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR