If an engine that ran recently now refuses to start but cranks over normally you generally break it into two areas, ignition or fuel. You seem sure the fuel is not the issue, so we go into the ignition side. IS THE THROTTLE IS NEUTRAL? I would start in the "middle" of the system, pull the cap & inspect the cap & rotor for moisture/corrosion & replace as needed. Then carefully test for spark from the ign. coil output lead to ground, if strong spark, check at the end of the plug wires. I would also tell my customer that if a boat of this age came into my shop with an unknown service history & I find the first evidence of a needed tune-up I won't waste my time troubleshooting until after I have replaced the plugs, wires, cap, rotor, points (if it has them) fuel filters, etc. and eliminate all the small contributors of problems. These motors are very reliable with minimal maintenance.