Well, it's back to the basics : air, fuel, spark, compression. My first question would be, do you have the OEM gray ignition coil ? They have the reputation for failure, my original coil quit just 2 blocks from home, gave no warning, but still produced a yellowish spark at the plug, but not enough to fire the fuel air mixture in the cylinder . Don't know your mechanical expertise, and the maintenance history on the bike, but everything is suspect on a 26 year old bike. You could start with checking the valve clearance , then compression, I would take a good look at the ignition system ignition control unit ( ICU) under the tank for di-electric grease between the ICU and heat sink, coil as mentioned, plug wires, plugs, then it's carburetors, if nothing has been done to them, rubber diaphragms, and other rubber parts in the carbs, throttle cables for condition, especially at the cable splitter under the tank. A good general inspection to see if something is loose or missing. No real easy way to narrow down the possibilities.
