Links to companies who will fit tyres to a BMW C1, or will supply tyres fitted to loose wheels.
BMW recommend a 120/70 R13 tyre at the front, and a 140/60 R12 tyre at the back and their recommended tyres are the Bridgestone Hoop, Michelin Bopper and Pirelli GTS23/24 but it's worth checking the forum to see what members think about the specific tyres.