What I would try first is when it's cold and first starting it pull the spark plug and ground it to the engine. Have a helper pull the chord and see if you get a good spark, if not replace the plug and do the procedure again.

Next once established you have a good spark use the mower and let it get warmed up. Now try to start it, if it doessn't start perform the spark test again.

if no spark it's probably the coil is bad and needs replacement, bad coils can fail to produce spark once in a heated state. This too can also affect why the engine has lost power.

Hard starting may also have to do with carb problems, make sure your primer is working properly and providing fuel when cold. If a fuel issue you can always pull the plug and pour a little gasoline into the engine, then quickly insert the plug and attempt to start. If it starts, then it appears the primer may not be doing it's job.

Can you do a leakdown test or find a dealer to do that for you? I suspect a leaking head gasket.