Quite a few people who have tried to 'roll their own' cores have fallen foul of patents held by the companies which developed the original device,I am thinking in particular of the 68000 and strongARM.So not only do you need to know the device like the back of your hand but you need to know which bits are fiercly protected.Try writing an 80387 and publishing it,I should think intel would be on the phone pretty quick.