Well like I said, it would be really tricky, if nothing short of adverse. That chassis and it's cooling system was designed around the power envelope of the gt650m. The next step up, and probably the highest option you could use in your chassis, would be the gtx 660m. They're both based off the same exact GPU (GK107), but the gtx 660m runs at a slightly higher core frequency.
You can review the specs here:
http://www.geforce.com/hardware/notebook-gpus/geforce-gt-650m/specifications
http://www.geforce.com/hardware/notebook-gpus/geforce-gtx-660m/specifications
As far as finding a gtx 660m to buy on it's own... good luck! That will be very hard, and likely, pretty expensive. On top of that, you're looking at a meager 15% performance improvement. In other words, the time, effort, and cost to do what you want would not be worth it. I'd say your best bet would be to sell the laptop (as a gaming solution, the gt650m does a good job) and purchase a new laptop with a more powerful mobile GPU (like the gtx675m, hd7970m, or the ultra fast and expensive gtx680m).