You´d have to sail it out of Iquitos. I also wonder if the bikes they make there would have the horsepower you really need, since they are assembled for the local market. And while it is true that the departents of Amazonas and Loreto are IGV 18% exonerated, I wonder whether the savings would offset the trouble of beginning your trip in an awkward location? Also bear in mind that any imported good they sell there will already be priced in with a premium, since the good needed to be transported there in the first place.
I would either buy in Lima, or in the fairly large coastal city of Chiclayo, in the North, so you can cross more of Peru.
Disclaimer: I don't ride motorbikes.