According to Leroy's Ramblings (great site for small HP OMC owners), the 9.9 and 15 hp models only had the different carbs form 1974 to 1978. From 1979 to 1980, it was a different carb plus shims on the reed valves. From 1981 to 1992 (the end of the series) the differences were the carbs, reed valve shims, and the tuned exhaust. Aside from the model plate on the motor, those should be the only real differences between the two. The power comes from the 15hp having a WOT operating range of 5500-6500 (possibly 7000) and the carb, reeds, and exhaust tube help achieve those rpms. For any mechanically inclined person, changing the carb and adding the reed shims or replacing the reeds altogether with aftermarket ones should be a piece of cake. I would also imagine the exhaust tube swap wouldn't be too hard. It would require pulling the powerhead but really that in itself is very simple, the hard part of that job is disconnecting all of the linkages and other components that allow you to pull the powerhead and get to the powerhead bolts.

As to finding these parts, Ebay is probably your best bet. You may be best off to also buy an entire 15hp motor that is shot and remove the parts yourself as I think people try to part out the 15hp conversion parts at a premium cost.