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A field of order p^2 may have different choices of generators. To force a choice, you can specify the modulus for the field. According to the documentation for GF (type GF? to see it), this is an optional argument with which you can specify "a defining polynomial for the field."
So you can do
sage: k.<i> = GF(431^2, modulus=x^2+1)
sage: i^2
430
sage: i^2 == -1
True
sage: E1=EllipticCurve(k,[0,161+208*i,0,1,0])
sage: E1.j_invariant()
364*i + 304
