You have multiple problems. Need to analyse them separately.
If turning on the deck with no other changes kills the engine, it could be something on the deck causing the problem. Such as something wound around the blades, i.e. wire, string, or piece of wood blocking them. Or it could be bearing in a spindle has failed locking it up. It should not be the clutch as all turns when engaged.
When did you change the trans fluid and filter last? If filter is full it might cause slow hydraulics. Have you been using the right fluid?
Is the engine "bogging down" simply because the engine has not been serviced, i.e. air filter, fuel filter, dirty carburetor, etc.
You need to do some thinking on what the problem really is.