When I was chief engineer back in the 90's at Torque Developments PLC , the Jap car specialists we imported suspension kits from all over the planet and was the HKS main dealer. What was not available from the spring , ARB suppliers like HKS , Koni , Eibach etc for vehicles we had them made or made ARBs ourselves from EN34 spring steel and had them heat treated , or modified coil over types to accept race springs .
I modify shocks by draining them and installing high pressure Schrader valves after adding bosses to accept the valves. The shocks can then be filled with the measured amount of oil drained with either a standard viscosity or higher viscosity oil and gassed with nitrogen or air ( air is mostly nitrogen and makes little difference in its function of suppressing foaming ) at 1 bar or higher as is required depending on final use via the new Schrader valve. The sealing system in KYB or other quality single or double tube shocks is normally pretty good for a so called '10-20%' or higher upgrade in bump rebound performance.
I run in a limited formula sport and have used this method to alter the original shock rates with great success and standard Chapman Strut type coil over types receive what are called steel bearing collar shaft adapters weldedon in order to fit more common size 2" ID diameter race springs of various rates . My race car is built for short oval and not one corner has the same spring/shock rate on either 'axle pair'.
Just a thought should nothing be available for this model in the he future .