OK so you need some help. Well first thing you need to do is identify the problem. As with anything else it involves trouble shooting. Just because it won't start doesn't really help you in finding the gremlin.
We need to figure out why it won't. Such as.
Step one. Does it have fuel. (Don't laugh everyone has done this.)
Step two. Is it getting that fuel? (Many mowers have a fuel filter and they do get clogged with stuff, and it happens more now with that ethanol shit!)
Step three. Does it crank? When it cranks does it have compression? (Compression is the resistance you feel if you try to crank it and it pushes back on you)
The list goes on and on as one can imagine. You start at the cheapest and easiest way to make it work and then go down the line.
Now a good thing to do is know what the fuck you will be working with. These are called engine diagrams. They are mother fucking invaluable.
You only need to Google the make, model, and HP rating of the motor you have.
For instance mine is a Briggs and Stratton v-twin 18hp. Don't need to listen to the hype. The ones made today are all the same for the most part in quality.
This is the cylinder assembly
This is the manifold and assembly
I have one of the flywheel assembly as well, but you get the idea. I am sure there are higher resolution, but these were just easy to find to show you. They show part numbers as well.
If it would be ok, I ask someone smarter with wiki or a resource page layout could help so it can be linked in the sidebar.
Anyways hope this helps.