What are u using as a Speed controller? If it is the one that came with your Rocket Racer than you are going to need to upgrade before installing a 180 motor.
As far as "converting" to a 6 cell, I am not really farmiliar with the chassis of the Rocket Racer but parts from the ABC DTM SP should fit. They are availble from Horrizon Hobbies or you can always ask Tiny.
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-Engineering Manager
Scar, you can get the 180 motor for the Rocket Racer here. Any 7.2v pack for the Micro RS4 will work. Or, you could go to www.blindbatts.com and order an MRS4 pack that has been tested in DRR's before. It is true that a DRR stock ESC will handle 7.2v and a 180 motor. If you want to go any higher though, you will need a better ESC. I recommend the Team Orion Flash. It looks pretty good and is tiny.