1. when you downshift (and not mean too) and over-rev your engine, you can break a number of things including valves, piston rings, and other various things. When you rebuild you will need new valves, valve springs, valve locks, valve retainers, pistons, piston rings, connecting rods, connecting rod bearings, and new main bearings, as well as main studs, and head studs, and new gaskets for the whole thing...That's the major stuff...but there are quite a few smaller things that are in the head / valvetrain that you would also possibly need too...oh and a machine shop probably to help re-work the cylinder walls, and deck surface of the block