Old mowers take time to keep running but will run for a very long time.
New ones require very little maintenance and usually have no adjustments so they last a very short time.
You really need to get an operators , parts & service manual.
Most of them are on the Simplcity web site for a free download.
When you come to a problem look read ask then buy parts.
The brake rotor was fine it would have cleaned up to be servicable, I have never replaced the pins, again some wire brush work & they will be servicable.
All that ever gets replaced is the pads and there is nothing special about them I cut them from a long length of motorcycle brake pad material I bought 30 years ago for relining shoes on vintage motorcycles.
Remember it is a lawnmower, not a space shuttle.
If you keep replacing servicable parts it will end up being very very very expensive.
patience costs nothing, impatience cost a lot.
Same story with the gearbox these manual gearboxes are rebuildable and in most cases it is nothing more than the shift keys because people do not bring the mower to a standstill before changing gears.
Because Peerless no longer exists, most of the big parts companies list all the parts as NLA but in reality they are available from a lot of places, particularly in the UK .
These old tractors are really a single speed mower.
The box is used to select the speed range that will be suitable for what you are doing and you stay in that gear.
So when you come to a problem, ask first before you start ordering parts we will probably end up saving you a lot of money.
As you have already found out pictures work wonders particularly when English may not be your first language because for the bulk of the people here English is their only language.