hi 46Sparky,
a smaller value like 0.56 uF would make more sense than the information i found about rebuilding Lucas Klaxon horns elsewhere
a 15 uF in the 50-100V range in the old horn would have been a much larger physical size, so either they are mistaken or the circuit only requires a minimum value and they are doing over-kill.

0.56 uF will be a smaller and cheaper part, more common:
https://www.amazon.com/TOOGOO-Metallized-Polypropylene-Capacitors-0-56uF/dp/B075XC96CF
https://www.amazon.com/Solen-Metallized-Polypropylene-Capacitor-0-56uF/dp/B005TLXKPW

the old one has no markings on it? do you have a way to measure it?
sometimes the old paper caps can miraculously be good as long as you have a way to measure it's capacitance and leakage?
i've had luck baking the parts overnight at elevated temperatures, go hot, well over 150F, like closer to 200F, drive out all moisture.
-s