The LA145 has a sealed transmission. you can't do anything but cry if the tranny is slipping or wont go uphills. 600+ for a new one and 500+ to put it in.
It's not a 'lack of power'. It's the hydrostatic transmission limited on how fast it rotates to power the drive wheels
((I assume you mean beyond the highest throttle setting (using the throttle leaver).)) The lever should be attached to the carberetor by a cable. It may be possible to replace this cable with a longer one, but if so, it will allow you to run the engine a lot harder than it was designed for. You can wreck your engine this way (potentially in a spectactular and hopefully not life threatening way) in addition to voiding any warranty you may have. If you do wreck your engine, most likely you will hear a loud *clank* and the engine will stop instantly. The engine will need *major* work done to it to bring it back to life, assuming it's worth the expense. I really don't recommend 'overclocking' your engine.
lower the deck all the way down. at the rear of the deck you will see a plastic wingnut on either side. remove blades using vice grips to loosen the bolt. you will needto use a shop rag with your free hand to hold the blade in place so you dont cut yourself. next remove wingnuts in rear. there is a bar in the front-most part of the deck held in place with a cotterpin. remove pin to remove bar. depending on your model of mower, that should be it. if not look at the deck frame for any other nuts or bolts.