> If I was porting a game, I'd at least want to make important behaviors 100% true to the original - accelerations, jumping physics, hitboxes, speeds... these are pretty tedious to reverse-engineer without source.

You're wildly overestimating how professionally done a lot of video game ports were in that era. It wasn't uncommon for home computer ports to be written by a single developer whose only access to the original was being able to pump a pocketful of quarters into it at their local arcade.