There are three main issues that you will need to deal when adding a alternator to your engine
First -- you will need spare Horse Power from your one cylinder engine. It take about 1 HP to generate 60 amps at 12 volts. This HP is in addition to what you need to propel your vessel.
Second -- alternators need to be spun at around 5000 rpm
to operate. Thus, you will require a pulley, belt, and mounting solution that will run the alternator at the correct rpm
Third -- you will need to reconfigure your electrical system
to provide and control the field voltage needed to power the alternator.
I think you would be better off taking to starter/generator to an auto-electric shop and having it fixed.
Best of luck,