I think hagler wins easily but I believe he would fight this fight similar to how he fought duran. Hagler would box him silly, and every now and then put some leather on him. Hagler is far too technical of a fighter and his chin was too good to be koed by anyone at 160. Hagler wins a fairly borinf but easy fight.
Rocky has the right hand lead which can be useful against a southpaw. Didn't save him from being retired by WW leftie Davey, who moved away whenever Graziano swung it, and kept his right in Rocky's face. Marv's right jab is one of his power shots, and he'd be more than comfortable moving away from Graziano's wrecking ball on chain. Rocky possessed a measure of late round stamina many sluggers of his ilk lacked, but he could be badly outboxed, and didn't have the first rate chin necessary to absorb too many of those Hagler jabs. This would be another three round wipe out like Zale III and SRR. Marv would be stepping into that right jab, Rocky would get stunned at some point, then Hagler's finishing skills would conclude matters.