You might be able to narrow it down by the carburetor.
-If it has a throttle stop on the carb its an 18hp
-if it doesn't have a throttle stop it could be a 20-23hp
-if it has an accelerator pump on the carb it will be a 25-27hp
-if it is a 2bbl it will be a 30hp
usually the larger engines 23hp and up had the SAM ignition system because they were pretty common on commercial mowers, the little black box bolted to the blower housing.
Hope that helps out without tearing it apart.
it does have the throttle stop. so does that make it for sure a 18 or could it be smaller? and I didnt want to take the engine apart to do the bore x stroke but ya no numbers on the plastic cover/shroud, no numbers on the valve cover or anything. Also no black box, accelerator pump, so I am at a 18 hp. thanks.