Yes sure if you have the knowledge to make this you can. of course Unity is Open source ( if you pay i think ) But alot of people "REMAKE" thing who was already included in unity like : Collision, GUI, etc..
So im sure you can
answered May 16 '12 at 07:31 PM
Collision detection: sure, a lot of work but possible
Particles: yepp, if you don't want anything too fancy
Rendering, Lighting and pretty much everything else that needs low level access to the gpu (or at least GL/DirectX): I don't think so.
Question is: Why would you? If you wan't to build your engine from scratch, why use Unity in the first place? Plus there's only very few things you can't achieve using Unity's built in Rendering/Particle/Physics/whatever systems.