Charlie.
Sit down, take a few deep breaths and tell us the whole story, from start to finish in the order that things happened.
Also why you did things like replaced the deck belt.
It will help greatly because we have nothing to go on but what you have typed in and the more information we get the better it is for us and the easier to help you.
If you have the PTO lever on the right side mudguard and the blades do not turn off then some thing is missing from the deck, the return spring is on the wrong place , the belt is routed wrong or you have fitted a belt that is the wrong size.
Thus we also need to know what belt you fitted and where you got it from
Does the PTO lever move the same distance as it used to before you changed the belt.
Try removing the belt and see if the engine starts
