The black cranks that were installed on my Thorn Nomad and Thorn Sherpa are just Andel TMS1 (a generic brand) with Thorn branding applied. (The TMS1 logo is on the inside of the drive-side crank arm, if I remember). If your silver crank is a similarly rebranded generic crank, you might be able to find the missing left crank arm for cheap somewhere.