P0306 Cylinder Misfire Diagnosis for 1999 Buick LaSabre - Common Causes After 2-Year Storage

I recently encountered Diagnostic Code P0306 indicating a misfire in Cylinder 6 on my 1999 Buick LaSabre. The vehicle was purchased from a neighbor who bought it new and has only 60,703 miles — otherwise, it runs perfectly. The car had been sitting idle for two years in a garage with no engine activity. When I attempted to start it, the engine turned over but did not fire. After spraying starter fluid, it started briefly and ran roughly for just a few seconds before stabilizing. The check engine light activated while driving across the street to my home. Given its long storage period, I suspect the issue is related to fuel or ignition components. While I’ve read several forum discussions on P0306 in 1999 Buick LaSabre models, I’m not a mechanic and am unsure where to begin. I considered checking Cylinder 6’s spark plug — however, it's located at the rear of the engine and would be difficult to access without removing parts. Given that the car has been idle for two years, I believe a faulty spark plug or ignition wire is most likely. I’m hoping it’s not a more serious issue like fuel injector clogging. Any insights into the most probable cause? I’d appreciate expert advice on how to proceed safely and effectively.