Easiest swap will be putting a 7k in it. Nothing needs to be changed. Lots of torque although not as high revving.
If you want more power, you can try the ever popular 4age. Since this is an efi engine, it won't be as simple of a swap as the 7k. You will need (among other things) mounts for the engine and modifying the fuel tank. 3sfe is also a cheap and easy option.