The F1 or as others have pointed out, the Broughton Resonant Filter Equalizer, will get you the vowel-y sound of the Wal low pass filter + resonance, but the pick attack is a unique thing to Chancellor's Wal sound. Either of those pedals will probably get you, IMO, 70% of the way to the universe you seek. The other, harder to obtain 30%, is multicoil buffering, pickup location, pickup blend, pick attack, and circuit sauce adding a bit of grit.
It would be a hard thing to get someone to free up as there's not many of them in the wild (maybe 50 of them or so), but the Broughton Royal MK is an homage to the Wal Mk series preamp down to how the pick attack stacks with the low pass filter's cutoff frequency. I giggled when I first plugged it in with my Stingray because it is so good.