Engine hardware — the fastener kits, single fasteners, oil drain plugs, dipsticks and distributor gears that hold an engine together and rarely get ordered until something is already apart.
Fasteners are not a place to reuse parts. Most modern European engines use torque-to-yield hardware that stretches permanently on first install: it is designed to be replaced, and re-torquing a stretched bolt gives you a number on the wrench without the clamping load behind it. Drain plugs are the same story on a smaller scale — the crush washer is a one-use item, and a plug that has been over-torqued into an aluminium pan is how a sump thread gets stripped.
If you are chasing head studs or rod bolts specifically, those live with their assemblies: see cylinder heads and head stud kits and connecting rods and rod bolts. For the full fastener range including ARP head studs, main studs and rod bolt kits, see ARP engine fasteners. Doing a service at the same time? Oils and fluids covers the rest.