How many AMP alternator for this application

You don't need an ammeter. Voltmeter is whats needed. I can't remember the explanation I was given by the people at MAD electric but you are on the right track. Most electric fans draw around 20-22 amps. I would say contact the folks at MAD electric for any and all electrical concerns. Very knowledgable and familiar with the older systems and how to correct their flaws.