Speep reduction is sufficient.
Otherwise also vehicles that have like 90% of their hp will also have their Vet disabled by a si ngle ATNade/Faust.
Speed reduction barely makes a dent. It still goes just as fast (maybe even faster) than it would without engine damage, and the ability doesn't last for 2 seconds either.
Sorry man, but considering how hard it can be to cause engine damage even when the other guy is downright reckless with his tanks (which should definitely be punished), I just can't support leaving it as it is. I use it too when I play germans, and it's way too powerful. Panzers are solid enough as they are, they don't need a secret, uncounterable Hyperspace button on top of that. I don't care that it's a vet ability, everything in this game should be counterable by good play from your opponent. There is no proper counter to Blitzkrieg, you just let out a sigh as the Formula 1 Panther magically ignores those two mines it drove on yet again.