This is one of the games I used to play on my (pirated) NES/Famicom console before I graduated Grade School. Contrary to the title name, you play as Ultra Magnus, then Rodimus Prime upon collecting hidden letters in the game.
It was hard, even for a game that is filler to the G1 storyline (to the Japanese), and I finished it a few times. (WARNING: Epileptic lights)
Apparently this game is still out on the Virtual Console. And, it is remade for the IOS/Android for the Japanese customers. Couldn’t remember the name of the remake.