First, I apologize for not even READING your post! I saw that it was a power-vent (or, at least, I presumed it was a power-vent with a name like Power Flex) and shied away because I know so little about those.
The powered anode would be a reasonable solution if you can install one and not have the power-vent housing get in the way.
You didn’t say what brand the heater is, so I can’t go to the maker’s website and look at the user’s manual and get a look at it. That would help.
Usually, the anode port is about halfway in from the edge to the center if the heater has a hex anode, and insulation COULD cover it. There are other ports more toward the edge that are where they blew in the insulation.
There is no way to be sure of the heater’s status until you inspect the anode. Then you can decide if you want to proceed with a powered anode or new heater. The rule of thumb is that you replace an anode when you can see six inches of core wire anywhere on the rod. If your rod is down to the core wire, that would be an ominous sign. You could also post a picture of it here, if you wanted, and I’d tell you what I thought of it.
And yes, you can remove the powered anode from a broken tank and put it in a new one, provided they have the same type anode: hex or combo.
Again, I apologize!