3.5hp will spin a Chrystler 70 amp Alternator....
Get an old one from the 70's with an externaly regulator, these realy charge well, and they're simple. Add an on/off switch for the field so its easier for the Petter to start and get up to speed.
A 3 HP Briggs and an alternator like this charged batteries at my camp for 20 years. You don't need a fancy regulator. Never let batteries go below 80% charge if you can help it and its less strain on the charging system and your batteries will last longer.
Doug