The Bultaco screws are OK but I wouldn't recommend using standard self-tappers because they are too pointy and sharp and rip the bead to shreds within seconds of getting a flat. Concurrently, the steel wire from the bead winds up around the axle/sprocket and things come to a stop.
My advice comes from first hand experience in 1977 or 78. It was somewhere near the Glasshouse mountains. After getting the remains of the tyre off and unwinding all the wire, I rode to the next checkpoint very carefully on the rim. I remember the day well also because Phil Eldridge stopped and asked did I need any help and I was quite impressed to see someone racing with a prosthetic leg.