The Power Supply's are different sizes, a 17" might work in a 20", but definitely not the other way around. when you sad "internal guts" I hope you don't mean actually opening the Power supply..
if incase the big warning that says "DO NOT OPEN, DANGER OF ELECTRICAL SHOCK!" wasn't good enought for ya.. let me say.. DO NOT Open a power supply.. the capacitors store power even when its unplugged.. and we're not looking for "darwin award" nominees here.