It's not the same since Apple stopped sublicensing manufactures like UMAX. Though the licensed "clone" machines were allegedly not as solid as the ones that Apple built.
Basically, you could create a Frankenstein computer, hack apart parts from other machines and combine them.
I don't think it would be worth the time or effort.
Most everything about Macs is proprietary, unlike the WinTel world. This is one of the reasons that Apples (regardless of OS) are more reliable than equivalent competitors. Parameters are tightly controlled and the OS and processor seem to co-evolve, and thus are integrated better.