Scizor is indeed an option, but if spinning is needed in the same slot Forry can do a much less offensive but good job.Just use Scizor. He can take at least one good Draco Meteor too, which is especially useful considering Ninetales attracts Dragon types like flies to a bright light.
Venusaur has STAB Sludge Bomb to OHKO most Dragons at +2, STAB Grass to kill Garchomp, and EQ for Heatran (probably not since Balloon is the shit now, but you can cut your losses and switch out on that one). There aren't really any common Scarfers faster than him either.
Shiftry has Dark Pulse, which OHKOs most Dragons at +2, Low Kick to OHKO Heatran at +2 (and Sazandora, the only Dragon Dark Pulse doesn't cover in OU), and Hidden Power Fire to OHKO anything those moves won't cover.
Exegguttor is really slow, but fast enough to be a sweeper; it learns Low Kick for Heatran and can put most Dragons to sleep or KO at +2 with HP Ice. It'd be a problem if Sleep Clause was activated and you opted for HP Fire instead, but then again...switch.
Venusaur, Doredia, Tangrowth, Victreebel, and Exeggutor have access to Sleep Powder. The only one I'd say is anywhere close to unviable is Doredia, simply because its movepool is awful and gets walled by something common no matter which HP it chooses. Butterfly Dance is really tempting, but you'd have to do some hardcore clearing before you can get a good sweep going.
Aye, all do have options to beat a lot of the aforementioned Dragons and Heatran, but none can beat all of them on the same set without sacrificing power or coverage. It's somewhat similar to rain in that matter in that the sweepers can be tailored to beat each other's counters, but I feel that the lack of sheer power as opposed to the Rain boost is what would make the Sun sweepers less broken in theory, but in any case that point is not here.
Also, a lot of the sweepers rely on Growth in order to get KOs on the pokes you mention, which requires the free turn, and admittedly Kabutops is the same and Kingdra like its DDs, but something as able as Specs Kingdra to come in and off the bat fire off immensely powerful attacks at great speed is a resource Sun lacks.
Also, confusingly and disappointingly, Exeggutor does not get Growth, so his use of Hp Ice at +2 to beat dragons is impossible, and is in fact one of the reasons why I would place him with Doreida as the only two of the Sleep Powder pokes you mention who are less viable.
Sleep Powder itself though is of course amazing except for its accuracy, but the problem with it is that it takes a slot otherwise better used for coverage or Growth.