Wow! I wanted this bird before, now for sure...P61,P61,P61!!
Boo
Keep in mind that the Mosquito F.Mk II had significantly lower performance than the Mosquito Mk VI we have in AH and how much additional performance the example in the duel was lacking due to being worn out is not known.
That said, if I were to engage a P-61B with the Mosquito Mk VI we have in AH I would not be turning with the P-61, but rather I'd be trying to use my superior speed. I am not sure what the P-61's climb rate is like, it has powerful engines, but is very heavy as well.