I think to roughly get what you want you can replace the [BENIGN] tag with [LARGE_PREDATOR], and then adjust the BODY_SIZE tokens to be bigger. I think that carp used to be bigger relative to dwarves, but I'm not sure, because as you noted the tags have changed a lot since then. The biggest difference is that they are now There is no "damage" directly assigned to creature attacks - the damage depends on the material of the bodypart used, and the size of the bodypart and creature. If you want to get more accurate, you could give the carp teeth, and make their attack use the tooth body part instead of mouth. You could look at hippos for an example. Actually, if you want something close to the deadliness of the old carp without much work, just copy the whole raws from Hippos to carp, and then replace the [BENIGN] tag with [LARGE_PREDATOR], and then your dwarves will learn to fear the water...
I am not an expert in modding, so I don't know how (or even if it is possible) to exactly reproduce the carp of old. As I said, my understanding is that they were larger, and were aggressive. Based on the raws, it also looks like they attacked using teeth (which they no longer have), but i'm not sure. In other words, they were the aquatic equivalent of elephants, which I why I suggest you just copy hippos (and make them aggressive) if you want a quick simple change.
Also, I am not 100% sure that [LARGE_PREDATOR] will give them the proper level of aggressiveness (if anything it may make them too aggressive) - you might just need to remove [BENIGN]. But I think it should make them attack like wolves (which also have [LARGE_PREDATOR]), which sounds like what you want.