It can be rebuilt, but it is labour intensive whether by a fabricator or a DIY project. The most difficult issue is dismantling the door without damaging the inner and outer panels if they are still intact.

The original wood core has to be totally removed, the aluminum treated for corrosion and a new core installed. Do not use pressure treated plywood. It will ruin any aluminum against which it is placed. Baltic Birch plywood is among the most stable and strongest plywoods available. I have rebuilt ramps using this product with excellent long term results.

The wood can be epoxy bonded to the aluminum skins with marine epoxies such as System Three, WEST, MAAS etc. The epoxy will be thickened with various fillers to the consistancy of ketchup, and will bond the metal to the wood. It will also waterproof the new wood and eliminate any future rotting.

This door delamination issue has been previously discussed about the KB trailers. You might be able to google the older threads. If I'm not mistaken, after market doors are available, but at a substantial price. With some basic materials, aluminum sheet metal, plywood, epoxy and fasteners, rebuilding them could easily save  a great deal of money.

Here's an older inquirery that might offer some answers:

http://www.horsetrailers-online.com/forum/thread-view.asp?threadid=9838&posts=12

Wow- I am honored to be in the presence of the Beacon of light! Even if it does overshadow everyone else..... Was that to be a complement to us?