If they don't pass emissions, they cannot be legally imported. It doesn't matter whether you want to use them on private property; that private property is still within the US, and subject to EPA regs.
People think the same thing about not having to register their snowmobile, ATV or whatever if they're hooning around on their farm or private property. Nope. State environmental cops will happily step onto your property, chase you down, and cite you.