One option would be to sell the bike, and transfer the title, while keeping the original title as well in the first owner's name - so you now have two titles, both in different names (or at least a certified bill of sale). Jump on the ferry from Buenos Aires to Colonia, Uruguay with with bike and both the previous and the new owner. This way the previous owner can export the bike from Argentina in his/her name - so no problem with customs - and when you arrive to Uruguay the new owner can import it into Uruguay in their name. At this point the new owner has the title in their name, and is in Uruguay with the importation certificate in their name. From then on no problems, either travel in Uruguay or turn right around and take the ferry back to Argentina, importing the bike this time in the new owners name with the current title.

Note that this only works for non-Argentinian/Uruguyan citizens. Nobody can import a moto temporarily into their own country, since they're not there as a tourist. Nor can they do a permanent import without paying a huge import tax, and often it's not even possible at all, even with paying an importation tax. Shouldn't be any problem though for any foreigner.

If you cross on the ferry they give you 8 month on return to Argentina. When I came in from Tierra del Fuego in the south they gave me 10 months.

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www.motoadventures.org Photo Journal of My Alaska to Argentina trip.
www.casakiwi.net My Moto Hostel in Medellin, Colombia...with a garage of course.