A 10hp cover won't even come close to fitting (way to old). Check with the Salvage Companies listed at the top of this Forum page in the "Top Secret" thread.
If you think that a 9.9 cowling would disguise your 15 for use on 10HP limit lakes you might be very expensively surprised. Some such attempts to outsmart the law have resulted in confiscated boats and large fines.
If that is your motive I suggest that you find someone with a 9.9 who wants more power and arrange a swap.
