Unfortunately, this is one of the areas that a Noel trade would've helped us with.
KO is simply just not a championship-caliber starting 4, and unless you're going to convince Durant to play the 4 next to Horford, which seems very unlikely to say the least, you're going to have to somehow upgrade the 4/5 spot to seriously convince Durant to sign here. A lineup of IT, Bradley, Durant, Horford, and Noel is a much easier sell than IT, Bradley, Durant, KO, and Horford. Also, I don't think simply replacing KO with Crowder is a feasible option due to size advantages we'd be giving up.
So I'd try to sell Durant on teaming him up with Horford and us attaining Noel to shore up our frontcourt. If he'd agree to sign here with those moves, then I'd offer up Smart or Brown to the Sixers for Noel, preferably Brown since he'd essentially be needless now with Durant. If Embiid is still looking good, I think Philly would have to make that trade, because that would give them their future 3, 4, 5 in Brown, Simmons, and Embiid.