The 'metal bracket' you describe sounds like a locking tab washer ... the 'tab' is bent back flat from the nut to allow it to be loosened ..... purists would say to replace it every time but I've re-bent them numerous times in other applications .... check what it looks like at the bend and see if it looks like maybe this is its first life and it doesn't look weak and fatigued at the bend .... alternatively it may be able to be fitted in an alternative position so you can bend a fresh bit (or, if reachable, bend up at another flat once the nut's tight thereby using a fresh bit) depends on the design of the washer ..... as for the impact driver.... I tend not to use them on nuts on shafts disappearing into engines unless I'm sure what's going on in there ... there are horror stories of the vibration loosening things internally ... possibly unlikely, but I'd start with a manual spanner and work up to the heavy stuff if it proves un-cooperative (keep the cutting torch and dynamite in reserve for when it really gets you annoyed🙂.)
I don't know that engine in particular .... the above are just general observations from working on older engines of various types over the years.
K
