I was so bored this summer, I bought a pack of neodymium magnets to try with bib numbers. Holy mother of polarity! They worked great. On both a 60-mile and 100-mile event, I had zero issues. 1/2 x 1/8 Inch Strong Neodymium Rare Earth Disc Magnets N52 (24 Pack) | totalElement
I’m done with pins and Scotch 77 (?) which was a gummy mess for me.
For grins and giggles, my son used the same magnets for a ultra-marathon he ran this fall. Even with a hydration vest, the magnets worked fine and his bib numbers didn’t budge.
I have no idea of these magnets are ‘legal’ in various sanctioning bodies. I have no idea if they’re safe when pressed against your body for 5 hours. You might even get better cellular reception with them 
YMMV