Pooch,

The first step is to measure the damages. If all is okay there, for $1000 you can send it to England and have it reproofed at 3-1/2" assuming there is such a chamber length in 20gauge. If it is accepted and passes proof, no major harm has been done.

However, if it fails proof or is rejected due to the modifications, a valuable gun has become worthless in much of the world and with discerning collectors.