Both "corpora" and "corpuses" are correct. Sometimes we import the plural as an irregular plural in English; sometimes we don't. "Corpora" is common enough that you're not wrong to use it. "Corpuses" (the regularized English plural) gets enough use that it's valid.
"Corpi" gets used as a hypercorrection, but it's deprecated in favor of the two previous.
"Corpi" gets used as a hypercorrection, but it's deprecated in favor of the two previous.
"Corpii", as suggested above, is right out.