You pretty much answered your own question - Different engines result in different agility stats, because a portion of ship acceleration stats is defined in the engine, rather than the ship specs. Doing a proper engine change plugin would first require every single engine in the game to be standardized based on their class, acceleration stats of each ship reevaluated based on the engine, and every ship agility updated according to the new accelerations.

Doing that only for snubs would be far simpler, as their engines are already more or less standardized. The issue starts with gunboats and above, where the stats defined in the engine takes a more active role in defining the ship agility and acceleration, and also a lot of caps and transports share the same engine regardless of their class. It's a massive load of work, and considering the current to-do list we have to go through, I doubt it'd be happening anytime soon, and even then, if an engine change feature is actually implemented sometime in future, it'd most likely be exclusive to snubcrafts at first.

So yea, by "still not possible" I didn't mean it technically cannot be done. I meant ship and engine stats are still not ready for an engine change plugin, as a plugin like that would require a complete rework of every single ship and engine in the game.