Some of the big bore kits remove/disable the power valve, which can have a big impact on power, and often limit your usable rpm range. If you set it up for low end power (likely on a 250/360 conversion) it might actually make less peak power, but should have a nice strong low end.
The lack of power could be from a worn out piston and rings. If you don't have any compression, it will kill your low end power.
As for jetting, keep in mind the bigger displacement sucks in harder. That means more fuel comes through the same jet per intake stroke than it would take with the stock bore. This can kill your fuel mileage, but the stock jetting might just be in the ball park. Only way to tell is test and tune.
Does your spark plug show any signs of running lean/over heating? Is it black and overly rich?
Usually, the best way to get a big bore to live up to its potential is to have the cylinder ported by a good builder, to get the kind of power you want. A mismatch in components (low rpm set up for carb and cylinder bore on a bike with a high rpm port job, power valve open and high rpm pipe) can result in the worst of all worlds. No low end and no high end.
