The grossest thing I was ever able to salvage myself was a TRS-80 Model I Expansion Interface that had mice living inside of it. I didn't discover the until I'd powered it on and attemped to use the printer port. I got nothing but garbage, and then noticed the "mousy" smell. The "motherboard" inside the Expansion Interface is mounted such that it basically hovers in the middle of a three inch high box, hanging component-side down, and the mice had built a nest on the non-component "upper" side. Their caked urine and filth had shorted several lines on the printer port, producing the symptoms... it's amazing to me that the RAM and disk interface circuitry wasn't also affected. :^b
To make a long story short, the "clean it well with water and alcohol and let it dry completely" drill worked in this case. There was some minor corrosion of the board but it was designed with big fat traces so nothing was actually severed, and luckily the short in the printer port circuitry didn't permanently damage it. However, if the board had been mounted in the case component-side up and the same amount of filth applied to that side I suspect the damage would of been a lot worse. (Besides the chemical damage the mice probably would of chewed up the components themselves.)
I'm *guessing* the places that will cause you the most trouble even if there's not fatal damage to the board itself are the expansion slot connectors and the keyboard. I'm not sure what metal Apple used in the original parts, but I've seen some badly rotted/rusted PC/AT slots on motherboards that were just kept in humid garages, let alone outside. As for the keyboard, if any standing water got into the switches and rusted them out it might be a total loss.
But, hey, hope for the best. If the machine was sitting underneath something else (or upside down) and stayed mostly dry you might have a chance to get it running. If it was face up, half open, and home to a nest of squirrels it... might make a neat birdhouse!