Maybe you should clarify your goals/budget a bit for us. My Brat has enough torque for me and my budget, but the best thing for you could be simple routine maintenance or a Duramax swap, depending on your goals/budget.
Torque breaks things. If you want it, though, one very cost-effective option is to look for my Frankenmotor thread on this forum. My Brat's frame creaks and groans now because of the torque, and the brakes, suspension and transmission all need to be upgraded to make it less dangerous.
Fitting the DOHC heads in a Brat is harder than fitting older SOHC heads. Also, if you're serious about torque, you'd be foolish to not get some torque cams from Delta Cams. The drawback of having 4 cams is that you have to pay them twice as much.
Jacob