So, a problem I've had in Snorlax + MGengar teams is that Snorlax is only a very soft check to MZam, who also wins 1v1 versus Gengar if it comes to it. Adamant Snorlax is a 99% 3HKO versus Psychic (even Calm Snorlax is a 3HKO after Rocks), and Crunch is not a guaranteed kill even after Stealth Rock, which means Snorlax doesn't actually win the 1v1 versus a healthy Alakazam most of the time if it has to switch in defensively. Snorlax can no longer switch in at all if it has taken around 33% damage (as it is now a 2HKO). The lack of a AMuk (or some other Psychic resistance or immunity such as Melmetal/MGyarados) also means they can always Psychic with impunity, whereas they might attempt to fire some coverage (for less damage) if you threaten the AMuk switch.
In fact, that is a problem with Snorlax in general, not just against Alakazam, that makes me not in love with it. It fails to OHKO almost anything it is supposed to check, even if it has the proper coverage move (Earthquake does a max of 85% to Gengar, Rock Slide deals <50% to Zapdos on average...) which means once it is in 3HKO range it can no longer switch in.
I think in a metagame without Melmetal (which I assume is your case on cartridge), AMuk is the only reliable defensive Alakazam counter and should be on most teams, but I could be wrong. Snorlax does OHKO Alakazam with Facade if it is already status'd, so if you're positive you can get it to eat a status before facing Alakazam, the above does not apply. You could also attempt to beat last Pokemon Alakazam with an Explosion/Self-Destruct user (who could be Snorlax itself), and MGengar is a great Mega against almost anything other than Alakazam, so this strategy could work.
A final option could be Chansey, although it gets shut down by variants running Taunt + Recover and is less of a threat offensively in general.
There is a team in the Sample Teams thread that uses Mega Charizard defensively, as basically a Mew with better typing with a WoW, Roost, Dragon Tail and Flamethrower set. This frees Mew to run Stealth Rock and beat other Rockers with Scald. If you wanted something aggressive, this might not be what you were looking for, but the concept looks solid. Gave it a quick try, won my first two matches.
Here's the team:
https://www.smogon.com/forums/threads/lgpe-sample-teams.3645829/#post-8012727
Mod Edit: Combined posts