You do not change out horsepower per the weapons load.
What AH does is model one plane then give the weapons loads of similar planes (in this example of the I-16) because they are all similar to each other.
However, as far as I recall, the types in the different AH weapons options were all nearly identical, performance-wise. I did some checking way back when they first released the Rata, and it seemed they were all about par, and the only really distinguishable version other than this was the Type5 or 10 (sorry, don't recall which) which was the most numerous made and technically pre-war but served at the beginning of the war.
If you do some forum searching you might find some of the discussions about it when it came out. I'm pretty sure they said "the engine is this model" somewhere, with charts comparing them.