Another alternative is an HO alternator from M.A.R.S. KEN HEMING rebuilds Goldwing alternators up to, I think, 850 or so WATTs, and they are original parts, so plug and play on any GL1500 motor with no mods. I would search this item on the tech forum, as there have been some problems reported, but not many.
http://myplace.frontier.com/~hemi-roid/Fred.