if you can find wood bleach crystals there in Mexico, which are Oxalic Acid, and something like a kid's wading pool big enough to soak the frame. this may be your best bet to get rid of the rust and maybe salvage all sound metal.
Search these archives for Oxalic Acid topics and you'll find loads of advice, some might even be helpful!

best of luck, it's not a Merckx but looks like a decent Italian brand and a worthwhile project