Actually I just bought a $2 ferrous metal blade at the local big box
and cut away with no problems. I started with just a surface cut and
lowered the blade a little on each pass. Once you're through the top
you can lower the more than an inch or so to go through the foam until
you hit the bottom cladding.

The metal cladding at most is only 1/16" thick on both sides. Inside
the door is nothing more than what I guess I will call "structural"
foam.

I can hollow out some of that foam on the "new" door bottom and put the
real plastic structural piece back in with some phenoseal or somesuch
adhesive that doesn't disintegrate the contacting foam.

Then all that is left is trimming the wooden jambs, reassembly of the
threshold pieces and viola... installation.

How do you like me know... me and my bad self?