Shinjin: not to answer for the moderator but I can tell you there are more things you can do with a SMLE action than you might think. In the first place, a Canadian fellow named Elwood Epps created a entire family of cartridges based on the 303 British case and there is a lot of information about his cartridges on the net. I have no doubt that most decent barrel makers offer some of his chamberings. The SMLE, esp the #4Mk1, has been sporterized and rebarrelled to 45-70 by the Gibbs Rifle Company, and I think they are still in business.
Where are you that 303 British is scarce, because just rebarrelling in the original caliber should be a option. When you say 'sewer pipe', do you mean badly pitted? Have you shot the rifle to see if it shoots halfway decently - might save a few $ if you don't need to do anything. Sometimes old copper fouling peels off and really makes things look terrible.