The best way would be to use a bigger HP motor with a pulley probably twice the size of the one on the alternator so that you can probably run it at 2000 rpm and it would be spinning the alternator at 4000 rpm which is probably its max output rpm. The alternator may even peak out at lower rpm. I didn't check the one you put up but as it was direct coupled it may have been peaking out at maybe 2000rpm
