For me fixing anything old and mechanical usually means cleaning it first. If I'm really lucky that can be the fix in itself but usually it just the first step.
This is my current metal cleaning setup;
small brass brush for detail and smaller items
large brass brush for larger areas
large steel wire brush for heavy dirt and rust
Copper cleaning cloth. Used with water or a cleaning solvent these are amazing at lightly cleaning large areas of metal. The copper cuts through dirt but its hard enough to heavily scratch the surface of whatever you are cleaning.