I think there are still complete plug and play kits available that don't require any rewiring of your ski and of course are more expensive than purchasing an external RR for a 951DI for instance, and cutting/splicing your harness yourself.
It greatly depends on your level of experience, I think.
Google is your friend, search for "951DI voltage regulator" or "951 external regulator upgrade". [MENTION=41828]Minnetonka4me[/MENTION] can also help you with parts and technical advice, he's one of the good guys and his parts are proven reliable so you can trust he'll get you on the right path with the best solution.