I have a gas pool heater that has a 1/4"-18 NPT brass plug
that goes into a cast iron manifold. Over time the threads in
the cast iron have deteriorated and now I can't get the plug
in. I tried a new plug and one of the other plugs from another
location.

I checked to see if there is a helicoil for npt and they have it.
I'm not sure how the helicoil works for a pipe fitting so that it
doesn't leak, but I guess it must work. Problem is that so far,
I can only find the insert in packs of 12 for $25, the tap is $45.
Then I think you need an insertion tool?

So, any other ideas? It doesn't have to be the most 100%
reliable solution, ie if it drips a bit or fails, it's not the end of
the world. Also, I'm not even using the pool heater, but I'm
not keen on ripping it out either. The area of the plug just
has warm water going through it, not super hot. So, I'm
thinking to just epoxy it closed. In the Fall to winterize it I
can just knock it out. Any other ideas?