I had a similar setup on my sailboat. 4hp kubota diesel engine belt driven to a 60 amp alternator. The alternor wouldnt carry my load, so I put a 150amp on. Then the belt slipped all the time, so I went for bigger pullys/belt. Then at full load the engine slowed down and stopped. I know now that a little planning could have saved me years of trouble.
4hp will actually produce about 2kw
V-belts are called "fractional hp belts" for a good reason.
the system has to be built to handle PEAK draw, no matter what average need is
Your 3 1/2 hp engine will produce a total of about 1 3/4 kw. 900w is doable, but you need a 1800w alternator to avoid overheating the smaller alternator at peak power. 1800w divided by13v is about a 140amp alternator, and they dont usually come in 140a,. so you'll need a 150a.
A 1/2" V-belt wont pull this much load. You need to go to extra wide industrial V-belt, or better by far, a serpentine ribbed belt. Of course, pullys must also be upgraded.
Now the system works good, except if you exceed your anticipated peak draw. If you ask 2.0kw from the system the engine will overheat, stall, and/or find other ways to express unhappiness.
General rule for boating #33451: You will ALWAYS need more power than you think you will. Usually double or triple.
Probably wouldnt hurt to think in terms of what will produce 4kw.
In 5 years living aboard our Catalina 36 I know I've made every mistake possible in building marine power systems. The above suggestions will cost more, but figure what a cheaper system that doesnt work is worth.
Zeke