Kev, PLEASE do not try to clean this shotgun. Any "cleaning" will greatly de-value it. Just oil the exposed metal surfaces and let it be. I have seen many old shotguns ruined by cleaning. Also do not try to shoot it with some type of jury-rigged shell set-up. You never did mention the condition of the chambers and bores. Are they clean or pitted?
I see you are in Canada. I can refer you to a couple of Gunsmiths in Canada that can advise you of how to proceed, PM me for info,,,,ED