Like Iron Head said, check your spark, remove plug wire, install screwdriver and let it lay on the cylinder head(to ground) and have somebody crank it over, watch for a spark. No spark then mag/distributor problems. I have a fake spark plug they sell at the parts house that clips onto a bolt or something so you can check spark, have used the screwdriver or remove spark plug tricks hundreds of times though.
Air filter dirty? It would have to be plugged up pretty bad to cause a no start condition though. Found a plastic bag sucked into an air filter housing on a Lull once, they complained it would crank but never start til they killed the battery. Can't remember how it got in there but I remember saying, "unbelievable".
Fuel, yes, ran across guys on jobsites, won't start complaints.....gas in the tank usually helped with the whole combustion process or in the worst cases, diesel fuel in the diesel fuel operated machine instead of gasoline, seen that more than one person should. On those old cars I used to work on 60's models you could remove the air filter and look down in the carb and snap the throttle linkage and see gas squirt out of the jets, usually a good indication you had a good working fuel pump getting gas up to the carb, but since these are gravity fed no fuel pump problems. Might be a good way to see if you have gas in the carb though, maybe got trash in it, blocking fuel flow? Or something else I've seen tons of, clogged fuel strainers, filters. Don't have enough hands and toes in my entire house to count fingers and toes on the times I have found that as a problem.
Shawn