The first problem you need to address is the state of the insulation on the existing wiring. It appears that there is little, if any insulation on the wire on the right as it enters the existing old school pancake box. That is an invitation to a short that risks blowing fuses/popping breakers (at best) and electrocution or conflagration (at worst). Can't see the condition of the other wire from the angle shown.
If you cannot get a good wrap of electrical tape over the exposed wires before progressing, you will need to rerun a new cable to the boxes.
Assuming you can solve the wiring issue, you need to mount either the new fixture or a mounting strap to the old box. If there are screw holes in the box edge that align with the mounting holes in the new fixture (doubtful from the pictures), you may be able to screw on the new fixture directly. This assumes that the center stud/hickey does not stick out too far, and that you can get the mounting screws with the bare ends extending from the box to receive the cap nuts for the fixture.
If the holes do not line up, you need to use a mounting strap on the box, then attach the fixture on the strap. It is likely that the fixture comes with a strap that might work, or you can find universal straps that have multiple adjustments to fit most hole patterns.
If there are no mounting holes on the rim of the box, you may be able to use a center hole strap that can mount on the existing hickey. If all that fails, you will need a new box, and removing the old will further strain the insulation on the wiring.
Seriously think about replacing the knob and tube. Even if you an cobble together a solution with existing wiring, the insulation on the old wiring is inevitably crumbling and you are waiting for an accident to happen.
