I think another way to look at this question could be: What is the difference between "will" and "be to"?
As far as I understand, most modal verbs can be transcribed into the "be adj to" structure:
should → be supposed to/ought to
will → be going to
may → be likely to
can → be able to
must → have to
Therefore, "be to" can be loosely seen as the contraction for all the modal verbs above, which means it contains the meaning of "will" too. Since "be to" is a contracted form, it therefore has a more formal sense and relies on the context to determine its possible meaning(s). For example:
We "are to (will/should/have to)" meet at five.
No one "were to (could)" be seen.
But when "be to" is used in the past tense, it very often has the sense of "be destined to" or "be meant to" as well. For example:
He was never to see his home again.
Here, this sentence can be roughly seen as "He was never going/able to see...", where the result of his not being going/able to see his home was like a divine plan which he could not say no to. And I think the sentence in your question falls into this category. Therefore, it'll be grammatically correct if you change "was to be" into "would be", but the latter will lack the connotation/sense that his fate had been doomed.