You might be able to find an expanding bung that would fit into the wishbone that you could bolt the mudguard up to (by drilling a hole through it). Some MTB front mudguards work on the same principle.
I did this on my sons bike (the hole was already made by last owner) and used a bar end plug as the expanding wedge. Works a treat!
@MCamb started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
You might be able to find an expanding bung that would fit into the wishbone that you could bolt the mudguard up to (by drilling a hole through it). Some MTB front mudguards work on the same principle.