"the Surrender Letter I received said in plain letters that I may owe taxes."
Unless the letter indicated a specific nonzero taxable amount, that's probably just boilerplate text that they put on every surrender letter. Do you know for certain that some portion of the distribution is taxable? Only in that case would it make sense to submit a substitute Form 1099-R where you would have to enter in box 2a the amount that you believe to be taxable. In the absence of a Form 1099-R from the insurance company, it wouldn't make any sense to submit a substitute Form 1099-R with a zero in box 2a. In the case where the insurance company knows that the taxable amount is nonzero or they do not know if the taxable amount is zero (because they have lost the record of the amount of investment in the contract), they must provide you with a Form 1099-R.
If they do not provide you with a Form 1099-R, the implication is that they believe that the taxable amount is zero and, absent a reason for you to independently believe that some portion should be taxable, there is nothing to report of this on your tax return.
It's often the case that the surrender amount is less than your investment in the contract, in which case the insurance company knows that the surrender of the policy is nontaxable.
