The plastic bridge was really not a great idea in most respects but there is no doubt that some guitars wearing it sound just great. Guitars are like that, they sometimes don't care about things that humans get all lathered up about.
IME I'd not worry about losing the great tone of your old J-50 by having a well done RW bridge put on that guitar when the plastic one finally gives up the ghost. Many would claim it would sound even better, but if it sounds so good already, who cares? I do not think you'll find a 'reproduction' plastic bridge and wouldn't spend any time looking for one. I had a few of the 'adjustable saddle' wood bridge Gibsons and while they can sound just fine I'd opt for a fixed saddle because frankly a saddle requires very little adjustment over the life of a guitar.
For more detail on the Gibson plastic bridge & adjustable saddles find yourself a copy of "Gibson's Fabulous Flattops", a good book in any regard.
A nice RW bridge that is glued on properly will look good, transfer a lot of string vibration to the top--you kinda want that--last basically the rest of your life, and sound just fine.