I just replaced the water pump on my 871. I didn’t need the part number, I just called the DD dealer and told him what I needed. He had it the next day. Any factory auth dealer can hook you up. You might need the eng numbers though.
I don’t know about the 671’s, but on mine DD messed around with the gear that drives the pump. You can’t use new pumps on older eng’s without first replacing the drive gear on the cam. Luckily mine had been upgraded. They may ask you which pump you have.
I had to turn in my broken pump.