Might be worth a look at some of the newer snowmobile & personal watercraft engines. Some of them are in the same or higher hp range, operate in the 11-12K rpm range, and have a 1st stage reduction built into the engine to get rpm down in a more 'normal' range to feed a conventional gearbox. Single stage reduction from that rpm is probably going to be a problem to find.
Now that your question has been addressed (somewhat), how about project details?
Charlie