Well, I think you're right that you'd be hard pressed to find a better duo than the originals... I'm not sure that this is a better alternative, but what about the real-life couple Imelda Staunton and Jim Carter? I know Jim Carter has already been suggested for Aslan on a different thread but I think he might also have a Mr. Beaver in him - his voice has a very warm, earthy quality that might work. 

The Beavers in the BBC television series had very clumsy looking costumes, but I think the actors portraying them were quite good. It would have been hard at that time to have real looking animal costumes, but ridiculous as they looked I think the actors in them did give the animals some credibility.  The Beavers in the Walden films were pretty good although their dialogue wasn’t always completely accurate to the book. I found them very likable and their characters were fairly close to the way Lewis portrayed them. I think Lewis’ talking animals were among the best ever created in literature since they are so much like the real thing. 🙂

@narnian78 Dodgy costumes aside, it's interesting that in the BBC version, the Beavers both seem to have some sort of Somerset / West Country sort of accent. Again it seems to be going with the general concept that Beavers are the working class animals of the woodland world.

Even in American media, Beavers tend to end up with rural accents that sound to me like some sort of old "1860s Prospector" (i.e. like the Beaver in Lady & The Tramp) which is perhaps designed to evoke the idea of a Sawmill worker, or a Lumberjack working in the Frontier woodlands.

On the one hand I feel like Beavers as a species are hard done by with their media portrayal, on the other hand it's clearly a useful audio-short-hand to use working class and rural accents to sell the concept and meaning of a Beaver as a Tree Cutting, Dam Building worker.

Still, I can't help but wonder if there is a different trope that could be found there.

A production of LWW I saw in Canada a few years ago had a very suitably Canadian beaver! His accent, clothing, and gear were all suitable for the country we most associate with beavers!
BBC's Mr Beaver was actually a Canadian, and it's a shame the production didn't allow him to use his own accent, instead of a West-Country one. The 1960s black & white one had a very west country Mrs Beaver in one of the remaining excerpts.  (Mrs Beaver was played by Lesley Nicol, who Downton Abbey fans will know as the cook Mrs Patmore)

@coracle 

The Beavers were very well done in the BBC production with the speaking parts even if the costumes were awkward and ridiculous.  This was also true of the other talking animals. I don’t think the other animals had outfits that were terrible although they might have worked better for a stage play, which is what the 1967 version looked like. But people will accept something that looks artificial in plays more easily because they are expecting the background to be artificial and more theatrical than a movie or television series.  The Aslan puppet used by the BBC was actually kind of nice. I really liked it except for the mechanical mouth movement.  The Beavers had human mouths which looked rather artificial, but that was probably the only way they could have had people in costumes. I think the acting made up for a lot of what could not be achieved technically and especially with having realistic talking animals. It greatly helped to show their characterization.