FWIW, and if you don't have/can't afford an impact driver but have a center stand: with the bike on centre stand, wrap a ratchet strap around the brake pedal and centre stand and pull as tight as you can to lock the rear brake hard. Then, as Kev says, attack it with the longest bar/torque wrench you can.

Just as well this doesn't have to be done too often: the front sprocket is supposed to be torqued to 120Nm - the engine only kicks out 58Nm so I hate to think what extra stress is being put through the chain. (So fit/torque up the new sprocket before replacing the old chain)

As for the cush drive rubbers