Sir:
Your gun is a Rigby. It deserves respect. And a true professional, attuned to the finest in English guns.
That said, I'd contact Kirk Merrington. If he's backed up, he can send you in the proper direction.
http://www.merringtongun.com
And... such work as you desire on a Rigby is simply not possible to accomplish in a matter of weeks. But the result Kirk can provide will be more than worth it.