The brushes will be inside the alternator on the end oposite the input shaft. You will probably have to take the alternator apart to get to them. They must be free to slide in the holder and long enough to make good contact with the copper slip rings on the rotor [part that turns].
If it is an AC output machine it will not have diodes. If it is a DC output they will be mounted inside the case, same end as the brushes. There will probably be 6 of them mounted in a heat sink, or at least where they get incoming air flow. They are usually round with 1 wire each. Sometimes a blown one will have an obvious problem, other times not.
The biggest difference between this and a 24 volt alternator is the voltage regulator, it is designed specificly for a welder. Hehr Powerline in Texas and few other companies made welder alternators like this.