No, it is impossible to find. The only thing I know is that the last PlusFours can be ordered with a handeling pack, with Nitro dampers and a rear anti-roll bar. But nothing heard of seen about something to be retrofitted.
Back on topic.... dales_plus_four gave a (very)short reply to check the MorganWorks feed. I did and they refer to Plus Line improvements but any link they provided lead nowhere. So none the wiser. You can find Plus Line on the Morgan site but it refers to the new-new Plus Four cars (2024 on I think) and doesn't mention older CX-models. Happy to be corrected.
If anyone else finds anything please share it here
