The fixed idler can come off with the bolt in it as it is bolted to the frame.
The mounting bolts rust solid into the bearing so take it off, support the whole lot with a socket then tap the threaded end and the bolt will slip off.
When you have it completely out wire brush then grease the bolt or replace it
If you have another pair of hands handy it helps as the nut & washer is difficult to get at.
I seal off the fuel tank then remove the battery so this style of mower can be stood upright when doing primary drive work.
Not being a cow a face full of dry grass & dirt is worth avoiding.