REINFORCING a STERN

[Branko]

Hy!I have a small speedboat-It´s stern is rated for a 60 HP Outboard-Now I have a 50 Hp Johnson Sea Horse from 1975!The boat hits it`s top speed of 25 knots with it.I am thinking of reinforcing itś stern and putting a bigger outbard like a 85 Hp Johnson!Is it possible??????????????
My current outbard has 193 pounds-50 Hp
And the one that I`m thinking of has 245 pounds-85 Hp(also a Johnson)
My boat is 29 years old and has done very little time in the sea!Maybe 4 years when it all ads up!It was stored in a garage in a dark and dry place so that there isnt any sign of material degradation!
It is made of fiberglass!
How could I feinforce it and what would be the limit???
And how fast would it go with a 85 Hp engine????
Oh and my prop is 12-3/4" X 15"
So how could I reinforce it?With a steel plate,more plastic????????
PLEASE SOMEBODY ANSWER!!!!!!!!!!!!!!!!