The Norinco HP9-1 usually has a 14" barrel (though, sometimes, it is found with a longer barrel) and a 4-shot magazine and many parts designed to fit a Remington 870 will fit without gunsmithing.
The Norinco HP9 has a longer barrel - sometimes 18.5", sometimes 28" - and a 5-shot magazine. The 5-shot magazine means that a Remington 870 barrel - and a HP9-1 barrel - will not fit without gunsmithing.
Given a choice, buy a HP9-1.