First, a touch screen would not physically fit. If it were a desktop LCD, you could hang a resistive touch covering over it, but on a laptop that is not as easy.
Even if there were a touch screen that would fit, You would need a place to plug it in. That would mean re-purposing an external USB port.
I tried to do something similar: I tried to swap a keyboard from one Fujitsu Lifebook (the same model as what I use) into my Lifebook, to get the different pointing device. (I was trying to get a trackpoint instead of a glidepoint.) First, the pointing device was on a PCB that was in a different place from the pointing device PCB in my Lifebook. My Lifebook has no cutout in the (aluminum) frame where that PCB goes, so it does not fit. Second, the ribbon cable for the new pointing device is thinner than the ribbon cable I have a connector for. Too bad.
I'm sorry, it would be MUCH easier and cheaper to buy a new laptop with a touch screen.
--Andy