I have recently purchased a 1991 Sportrak ELXi. Only 45,000 miles on the clock. On my first long drive in the dark the battery gave out and went flat. I replaced the batrery with an approved Halfords one. Next drive in the dark it went flat. I suspect the alternator........ I have a used one from a breakers to fit. Are there any things I should look outfor? Is it an easy fit?