Its not worth fixing! Too much trouble! You don't want to even try! I'll give you fifty bucks and you can keep the motor!
No, really, I mean it! LOL
I would think you can just fill it like you were going to. You had told me there is no flex.
It is 34 years old! When it starts to flex you know what you have to do. If you don't trust it ,fix it now. Just imagine what the other ones would have been like if you had bought any of them. Mine for now is dry, mostly from the fact it hasn't been in the water for over five years.
The transom really isn't that hard to do. It could be worse, it could have wood stringers!