If it were up to me, I'd take 1.2 and painstakingly upgrade it beyond the scrapped 1.3 by not only re-adding all of the customizations from before 1.2, but also making various additions like adding new badges such as "Earth-Shaking Burps", "Bloat and Float", "Sickness Vaccine", and "French-Fried Thunder Thighs" (the last one in reference to SpongeBob's pre-blowup warning to Squidward in "Just One Bite") while also adding a fart toggle and related badges like "Mustard Flatulence" (which changes the gas to the archetypical Japanese yellow and the name being a pun on the common name of a deadly type of deadly gaseous chemical), "Muddy Fog" (which changes the gas to a poop-like brown), "Tremoring Toots", "Poot Soup" (which changes the gas to the traditional green), "Wideload Whiteout" (which changes the gas to plain white or reverts the color changes back), "Windy Waddling" (which causes gas to emerge with every step the character takes), and more, as well as a hiccup toggle and its related badges, more food items, some with quirky and referential descriptions (like a bag of cheese curls made via recipe referencing South Park's Cheesy Poofs, barbecue chips sold in the store referencing King Dedede from Kirby: Right Back at Ya! and the events of the episode "Fitness Fiend", and rice balls made via recipe referencing the infamous English dub of the Pokemon episode "Primeape Goes Bananas" where rice balls were mislabeled as jelly donuts), restoring the various room types (including the iconic "P!ss Yellow"), making it so that the objects in a room stay in place when first entering instead of being slung everywhere, adding more personalities for characters like "Villainous" (i.e.: characters like Vicky, Cartman, and Dr. Bittersweet), "Genius", "Snarky" (i.e.: characters like Edna Krabappel, Dee, and Meowth), "Focused" (i.e.: characters like Yoda, Dhalsim, and Garnet), and a whole lot more additions and changes.
(TL;DR: I know it would be super-difficult for me to reverse-engineer 1.2 alone, but those were my ideas of what I'd add and change.)