I have come across these Diana's in the flesh, both for sale and at the odd scooter rally. There is one place who might be able to help you as they sell these type of oddities, such as Heinkel, Diana, Triumph, NSU, Zundapp Bella scooters from time to time.
https://www.trojancars.co.uk/used/bikes/
Although they might not have parts, they might have contacts because they clearly renovate/restore these scooters from very tatty states. Trojan Cars are in Southsea, Hampshire, UK , right on the south coast. Worth a shot and maybe if you get tired with chasing parts etc could be an outlet for selling it on or trading it in? Im not connected with trojan, i just pop in there from time to time to muse over their vintage scoots and bikes.