First, the battery is with the keyboard side, not the screen side. And so the mainboard, CPU, ports, and so on and so forth. Basically, the screen side has just the screen, and nothing else. So there would be little if any savings in weight or size.
Second, the battery is likely to be "worn" and have little charge capacity left.
Third, old notebooks are often quite dificient in CPU and RAM for modern OS, like Windows 8. W8 is supposed to come in "lite" versions, but who knows how well that would work.
Fourth, ADDING touch screen is actually quite difficult, esp. if you want a proper "capacitive multi-touch" touch screen. I've seen older 'resistive' touchscreen refit kits for netbooks that's basically an overlay much like a screen protector, but capacitive touchscreen is glass and requires additional circuitry to process the touch signals, plus power for it to work. THEN you need to interface that with the PC itself.
Fifth, you still need a new case to fit the whole thing in.
There are some intrepid individuals who tried such a thing and I think I saw a few on eHow, but it's a fool's errand, IMHO, as they spent more than cost of a new tablet to build such a thing.