If you now have a good brake you do not need any replacements for now.
Cannot tell how much pucks worn when they are still in the assembly.
I would not buy any replacements until I completely lost brake (run out of adjustment) and then order AFTER the old pucks are accessed. If a brake puck access bolt twists off in the transaxle you are not going to need any pucks, You will need a crying towel.
You can do a search on-line and find a owners manual, pdf, for your machine with belt replacement instructions. The most general basics are:
The deck will have to be removed, the mower elevated, sometimes the front engine pulley has to be lowered or if the pulley rusted in place unbolt the engine and slide sideways
so as to get the drive belt off the engine pulley, while the clutch/brake is locked down.
You can also go to the Murray online SUPPORT site and use your model AND SERIAL # or contact Murray for owners manual. I just now seen this:
or see if this owners manual from Sears parts direct will match your machine pg 29-30
Here is a link to a parts list