Pull the old one out, it should be marked on it. Or. Measure the stantion diameter, the inner diameter of the leg and go onto bultaco uk site and match the size. The depth isn't to important as you appear to have plenty space.
https://www.inmotiontrials.com/?s=Fork+seals&post_type=product
I'd consider another gori or swm, better value than many other bikes of a similar age, and still very competative.
Lothian trials club have a trial this Sunday out past Gifford, worth a look of you have the time.
Any bike can enter, but the classic championship is pre65, twin shocks and air cooled mono - with a and b routes. All other bikes are in separate results.
I'm assuming you got that from Old Knobblies ? I heated and bent mine to suit, plus ground the frame marginally as required. I also tapped the head and fitted a decompressor to ease over tdc. This takes a huge amount of the effort out of starting.
Bear in mind some / all this corrosion may be due to your engine acting as a sacrificial anode, so it is equally important to make sure the frame is rust free and painted.
