I think that the first issue for the bumps and instability can be the wheel pressure, and you can try to go on problem until you fall down!
Another issue can be the steering dumper, if it is too hard you can feel a spring effect on the pocketbike, you can try to turn it on softly regulation.
For exchange the fork i think that you have to try pipes 1,5mm, not less.
About the pressure of the wheel, it depends by your weight, for the front you can try between 0,9 and 1,3 max for the front and between 1,3 and 1,7 for the rear.