The problem will be not just the wheel, but the fork sliders as well, to provide an anchor for the brake backing plate. Mind you, for all the braking power of a regular enfield SLS front drum brake, you could save yourself the bother, and just not fit the brake cable.
However, Hitchcocks offer a full TLS drum brake conversion kit for C5's, including fully built wheel with brake, new fork stanchions and sliders, mudguard and Avon tire, for 675 GBP. The part number is 90165. The text reads:
This kit converts the disc brake back to the twin leading drum brake fitted to earlier models. We have had owners wanting to make their Classic EFI look more 'Classic' and whist it is not the normal way to go, we must admit that it does compliment the already good looks of this model. It includes complete new fork legs, mudguard and stays, 19'' wheel, Avon tyre, and cable etc.
Edited by Blair - The kit includes both whole fork legs and not just the sliders.