That's a neat idea. There must be a good few cushdrives that would do. I remember seeing one in a clutch that would fit easily, but can't remember where. Suspect it was Triumph.
Cheaper and easier however to change out the Series C ESA for the Series D, as advised by KTB. I did - it is a far more robust device - and also took the advice of Simon Linford, to use the gearbox to avoid beating the ESA to death. It isn't proof against wilful abuse. That said, mine runs very smoothly even at 30 mph in top, and the Renolds chain is 13,000 miles old, and has been adjusted once. And no, it doesn't drag on the ground...
(Another bit of advice, this time from Trevor, was to avoid mainshaft seals. There's little enough thrust area for the ESA as it is, and reducing it further isn't smart. My Series C ESA, even without seals, was slowly eating the main bearing spool, and the chain was already 1/32" out of line.)
For sale: mainshaft seal kit, bought and never used...
Tom