When is was removing this board from the heatsink the back of this fully encased rectifier stayed on the heatsink. Should I replace it or try to insulate it again.
If the device is OK electrically, simply add a piece of thermal conducting voltage isolating material (Mica or silica gum) and continue using as nothing would happen.
The plastic has thickness that will leave a gap and heatsink compound isn't a good enough thermal conductor to transfer the heat. If you can't find a non-ebay replacement, you can use a fine sandpaper (400 grit wet) on a perfectly flat hard surface to sand to see all of the metal.
This search shows positive and negative rectifiers that should be a good sub. APT30D60BC — Octopart