I know it's probably not as effective as Jirachi because of a lack of key resistances, but I think I know a good replacement to the Rock Polish set.
Mew @ Life Orb
EVs: 4 HP / 252 SAtk / 252 Spe
Nature: Modest
Trait: Synchronize
• Fire Blast/Flamethrower
• Ice Beam
• Giga Drain/Thunderbolt
• Rock Polish
Obviously it doesn't pack nearly as much of a punch as Genesect did, and it doesn't have as many key resistances as Genesect did (being Pursuit weak really hurts), but it does resist Mach Punch and 100/100/100 defenses make it slightly more bulky than Genesect (even though its type takes away from that). Not only that, but Mew has the luxury of Fire Blast over Flamethrower, and the extra power is almost a necessity on this Mew set, but if you prefer the accuracy, then Flamethrower is fine two. Hazards are almost a must in conjunction with this set. If you have one layer of spikes, Ice Beam is a OHKO on YacheChomp. Stealth Rocks allow you to OHKO Dragonite, and make Tornadus-T an assured OHKO instead of a "maybe." Full hazards also let you OHKO things like Jolteon, ScarfToed, Offensive Jirachi, Starmie, Mamoswine, and many other important threats in the OU metagame. So Deoxys-D would be almost a necessary teammate, as would a spinblocker, perhaps Gengar, but then the rest of your team would have to focus on killing Alakazam. Anyway, it's a pretty effective set, and it pretty good at filling in the hole that RP Genesect left in our hearts, and more importantly, our teams.