As a postscript, with some help from people in the know I found out that it's basically impossible to 'nationalise' a used foreign vehicle in Colombia. That is to say you can't simply buy a used motorbike with, for example, US papers and transfer the papers to make it a legally-owned vehicle in Colombia in the longer term - necessary for me as I live here.
This is potentially dream-crushing in my case unless of course you happen to own a suitable motorbike with COLOMBIAN registration etc. that you'd like to sell, in which case, I'd love to hear from you. Or if you know some miraculous way around Colombia's protectionist laws on used vehicles...