Low End Top End???

low end, mid range, top end
are all referring to low, middle, and high revolutions per minute of the motor.
listen to the exhaust to tell in what band the motor is.
high pitched is high revolutions (top end), etc.
Usually bikes out of the USA receive just enough maintenance to keep them alive. I bought a KDX here in south america and it needed everything; piston, rings, bearings, seals, air filter, sprockets, chain, spokes, etc.
So to get your bike working good you probably need to spend as much repairing it as you spent to buy it. That's the first problem. The second one is finding a good mechanic. I asked the Kawasaki mechanic what training he'd received and he said "none". great. So I wind up doing most work myself.
But it don't sound like you have any experience fixing bikes.

We can chat here OK.
Usually the crank seals are shot and losing valuable pressure. That is most important, and next is the KIPS valves. I got tired of replacing them twice a year and so just epoxied them in the open position. Not a good idea being made of such a soft metal. After a year I found the left one to of melted a little!!! So I just replaced it last night with another broken one I'd saved. Third most impportant is how worn are the rings. Replace them if the end gap (sitting in the cylinder) is out of spec.
You have opened up the air box inlet I hope?
Before you replace the pipe you should cut across the belly of your stock one and remove the insulation and then weld it back together.
Also I was able to get more low end power from my bike by turning the ignition rotor about 1mm (at the markers) clockwise which advanced the ignition. But it makes it run a little hotter so be sure to flush your cooling system with radiator flush (I think its an acid) and use half distilled water and half antifreeze.
Also I found a high quality fine metal spark plug increased the low end crispness of the power. (see my signature)
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