Yes, it's toast. Don't ride the bike until you've replaced it as you can damage other things not designed to run at 20v.

Just about any Honda unit will do, and most aftermarket ones too. They pretty much all work on the same principal. As someone has said, a Blackbird reg/rect will fit and is a little more robust, so why not contact one of the aftermarket guys like M&P or MPS and get a pattern one?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball