You don't really need an ice type for the Dragon Gym, you just need an ice move.
Lapras can be a bit annoying/ hard to get, since it only appears in rippling water under the village bridge. If you've already been training your Simipour, just stick with that. It has higher Sp. Atk and Speed than Lapars, and though it's lacking in defences, it'll get the job done quicker (providing you've levelled it up enough).
There's no move that Drayden can pull on Simipour that it's actually weak to, but Lapras is weak to Druddigon's Revenge and Flygon's Rock Slide. The only benefit Lapras has is that it learns Ice Beam by level up, while Simipour needs to be TMed it (and I don't think you get the TM until after Drayden).
If you have enough red shards, you can get Simipour MTed Ice Punch in Driftveil. Though at 75 base power, at least it's 100% accuracy, unlike Blizzard. Simipour's attack is also the same as its Sp. Atk, so no difference there.
Simipour @ Leftovers/ Ice Gem/ Nevermelt Ice
~ Blizzard/ Ice Punch
~ Acrobatics/ Shadow Claw
Acrobatics would provide some general coverage.