My experience when I converted my 55.1 from 6 to 12 was that all I needed to change was the battery, coil, voltage regulator and bulbs. The 6v starter switch should be OK because switches care about amps not voltage and 6v systems have double the amperage as 12v. Some say you'll need to put a resister in line with the gas gauge but I didn't and had no issues. To be safe you probably should because mine might have been the "exception" to the rule.
Your starter and horns will be fine as long as you don't abuse them.
