OS 91 4 strokes are very good engines, have 4 of them and maybe have owned and traded off 4 more. First, try a new glow plug. If still won't run you need to remove the valve cover and see if they are functioning correctly. If so then check the valve clearances, get help with that if not experienced with setting close tolerances. Use fresh fuel and good luck. 5 gallons though not alot for one of these engines could be a little gummed up or a little wear on the valves. Find someone who knows these engines and should be working fine in about 30 minutes.