I will be installing the Dalton DBMT-1000-2 clutch kit, in the spring to bring my RPM back up to where it should be. The kit is pricey (not bad in $US) and includes a helix, spring and clutch arms. You can see my build in the builds section.
After several emails back and forth with Dalton, they did not recommend this clutch kit because it was not tested on a Defender, so now I have the new helix from the 2020 Defender and a high elevation spring in hand but so far haven't gotten around to the install.