In my travells through Mexico, C. America, Colombia and Ecuador, i have only had to produce my vehicle registration. In Costa Rica, i was asked for a title, but in British Colombia, your title is the registration. This blew over without a problem. Really, because there is no connection between L.America and any other country, all you need is a piece of paper which looks valid and important with the following info:
Vehicle Registration (in big letters)
Country of origen
Registered owner and address
Registration number
Plate number matching the bike
Year, make and model
Color
VIN
Displacement
*note* in BC there is Aircare, an emmisions test from which motorcycles are exempt. On my registration is written Aircare Expiry, which is left blank because i ride a motorcycle. Because customs officials understand the word expiry, this became a problem for me because it was left blank. It took a $20 bribe to the nice lady in Aduana to get this sorted out.
In reality, i believe a person could easily forge a document, place a stamp and signature and place it in a plastic sleave so that it looks official and get away with it. The officials have no way of knowing if it is real or not, and don't really care, as long as all the numbers match and the name matches the passport. BTW, i'm not condoning this and haven't tried it myself... yet. I just feel it would work because i use only photocopies of my original without any problem. My BC registration is the most un-official looking piece of paperwork i have! Also, i have it copied on my computer in PDF format so that i can change the info if needed, say i need to sell the bike and produce a vehicle registration with the buyers name on it.

... oh yah, definately get the international drivers licence, two if you can. You will be stopped at some point by the police and they will threaten to not give you your licence back unless you pay them the fine they are asking for. Simply refuse to pay and tell them you will pick up the licence from the main office and pay the fine there. This will usually get your licence back, but if not, so what? It's a small loss! but if you use your REAL licence, which is a very important document, your screwed and easily extortable! Never give the police your real drivers licence! Many people photocopy their original licence and laminate it for this purpose. If your gonna play the game, be prepared!