Gear ratio changes

[moneyhole]

It's a 2.5L 4 banger. The guy thinks it just a blown head gasket.
It is not a wise idea or does it really make a noticable performance differance on my motor.
Rear ends are changed all the time in cars like a 3.73 if you are looking to cruz or a 4.10 to get to the end of the 1/4 mile first. If it were your boat, would you do it if you could get the Renken, trailer, & titles for $640?
The only other good option I have is an identical outdrive as mine, used with trim pump arms and complete outdrive with gimbal housing for $500.
I have a brand new water pump for the lower unit already.