I've also got a 2005 I picked up earlier this year. Mine didn't have the oil mod for it's first 5 years and it still runs great.....though I just did the mod as extra insurance. J&T ATV offers the mod for ~$200 on ebay or directly through his website. So pull off the outer case cover on the right-hand side of the motor and check if you've got the mod done or not.....prolly not. You can go to YFZCentral.com or other sites to read up on the specifics of what the mod is all about and what to look for. Seems hit or miss whether the 2004-2006 quads will need the mod, but having extra oil squirting onto the piston's wrist-pin can't be a bad thing.
Hopefully you don't have to do much to get it going. I have a pdf of the service manual, so PM me your email address and I'll send you a copy.
If you do get it going, there are a few cheap and easy mods you can do like 'zip tie mod' to eliminate throttle hesitation, 'cam mod' for instant power gains and 'throttle spring mod' to aleviate thumb-throttle resitance.
If it doesn't have a case saver, get one imediately. Just more cheap insurance against a costly repair.
Mine's piped, jetted, no airbox lid, and after market dual-stage airfilter and it practically rips my arms off when I pin the throttle. I call it 'the beast' because the power seems endless and the limits of where you can go on a yfz become solely a factor of the rider's desire & ability.....and not the machine.
Good luck!