But don't replace it until you know why it fried! If it just stopped working, or the output voltage is wrong it could just be old age and wear, if all the smoke came out it's worth checking for short circuits in the electrics.
With the standard kit on board 35amps is enough. I've run mine with driving lamps (10A worth) as well without any problems. New ones are easy to fit but you might get away with a new brush set and regulator. There's instructions in the manual for testing alternators.
