Good advise NJ ^^^^
20W--50 V-twin specific oil and a quart of HD Formula + in the primary if the clutch is stock. It ok if the primary comes out milky from sitting, you'll be changing both those fluids again very soon. HAND TIGHTEN the drain plug and then 1/2 turn is all with a NEW O-ring.
Flush the brake fluid
Clean the fork tubes with 0000 steel wool and spray with WD-40 BEFORE they get compressed to remove any corrosion and save the fork seals.
I'll add that old oil in a sitting bike will smell like fuel, so don't panic yet. But you did just shove old gas through the injectors so.... they need a cleaning now. It's possible to remove them without an "engine rotate" but takes about the same amount of time. There is a front and a rear one just like a sporty, don't mix them up.
Buells HATE being started cold and revved in the garage, and will foul plugs immediately. If you START it, RIDE it.
2003 means the fuel pump is of legal age now and it wouldn't hurt to throw in a new one, check the FREE service manual for the procedure (Buellmods.com) and avoid the ebay replacements (wrong regulator). I suggest PM Barrett on here for a QUALITY replacement or re-build kit. Also easy to replace without an engine rotate, hang the bike to remove the rear shock and you'll have enough room to slip it out.
all the rubber bits are bad, check the date codes on the tires. More than 5-7 years and I'd toss them unless they were in a climate controlled indoor storage (doubt it). Google "Buell front engine mount" to see what a bad one looks like, you can still find them but it's getting harder.
Look for grease seeping out of the wheel bearings and idler bearing, they die a quick death and never in the comfy garage, swap them when you swap tires. Look at the belt while you're down there, deep cracks are bad and 2003 had a special small belt and pulleys that should be upgraded to 2004-up.
NOW you can look at why it doesn't idle. Do NOT "throw in a tune". you need to FIX the bike before you MODIFY the bike, and a "tune" is not the fix.
Get the Buelltooth that NJ recommended. Rev-mo.com has them. They use a bluetooth connection to communicate with your Android screen. That FREE diagnostic program ECMDroid from rev-mo.com or GooglePlay store to "check your AFV" to begin diagnosis after you've done the above.
Once you catch up on the neglect this bike has seen, you really should have a monster on your hands! Enjoy the ride and welcome to the forum:eagerness: