Rehydration existed back then, It was just more uncommon. I'd say 163-165 but thats speculation. Him being about 160 in the ring is more likely. He'd be a small middleweight now. But Hagler would also be a giant welterweight. The man was ripped to shreads and would definitely be called a weight bully if he cut down to 147. He looked bigger than Castano who is at 154. That could just be his physique though
Height of one, but he has significantly more bulk in the upper body and legs than say Josh Taylor, who is big at 140. He's a pressure fighter so it makes sense. He is also top 2 at 154, should have gotten the decision against Charlo the first time and less skilled and much less durable than Hagler so Hagler being top 1 at 160 nowadays isn't far fetched