Okay! Good looking skeleton, for everything, let's just break down a few different roles to make sure you cover all your bases and get you something swapped in for Porygon2.
So starting with Gengar, I'd highly recommend swapping Icy Wind for either Thunderbolt or Substitute, Gengar doesn't wanna stay in on much of what he'd be Icy Winding, and can't really pick up a lot of OHKO's with it.
248 HP / 44 SpA / 212 SpD
Modest Nature
for Rotom ^ I switched my specially bulky Rotom-W who ran that same set to that spread and found myself picking up a lot more clutch knock outs, the special attack boost really helps pack some extra punch.
Don't invest in speed in Gliscor, lol. The little bit invested doesn't really put him into a better place to outspeed much, so just go ahead and throw that full HP investment in there so he's nice and bulky.
I'd really really recommend a Ferrothorn > Skarmory for this, because defensive build Ferro works really well together with specially defensive Rotom-W and it'll give you a solid pivot.
Defensive Build (Ferrothorn) @ Rocky Helmet
Ability: Iron Barbs
EVs: 248 HP / 8 Atk / 252 Def
Impish / Relaxed Nature
- Leech Seed
- Thunder Wave
- Stealth Rock
- Power Whip / Gyro Ball
Very optional, and you can use the rest of my advice and ignore this point and the team will still gel fine, but I do highly recommend him.
Alakazam is good, I wouldn't recommend Encore though because there's not a lot of things that really wanna set up on him, so throwing Substitute or maybe something gimmicky like charge beam on him would benefit there.
I also would recommend Psyshock>Psychic because your team's overall physical threat is really low. You already have Gengar putting some special pressure there as well.
And then in place of Porygon, you really need some fire type threat for stuff like Scizor, Ferro, and threats like that, and something that can break walls and not have a super big fear of quickly getting taken down. Due to overall bulk and massive hitting power, I recommend Banded Entei for the job
Entei @ Choice Band
Ability: Pressure
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Sacred Fire
- Extreme Speed
- Stone Edge
- Bulldoze
He'll Help round things out pretty well for you.
Hope even if you don't take all of them that some of the suggestions stood out as either ones you'd like to take or spurred some ideas to try some different stuff. Have fun with your team man!