The motor that has a v-drive attached is a complete package and you would not be out there looking for a bell housing or something nobody had, therefore the complete motor and transmission is the way to go. There are three big pieces to this system: the motor, the transmission, and the v-drive. If the transmission part of the v-drive is the wrong ratio it may be less expensive to remove it and simply search out the proper transmission and insert it into the package. The last time I checked on trying to change a ratio in a Paragon it was less expensive to just buy a complete rebuilt Borg Warner and that's what I ended up doing.
The other issue is, if you need a 1:1 ratio and the transmission is a 1.5:1, it may be possible to compensate with a higher pitch propeller, but that can get expensive too if it has to be a custom job.
Taking 4000 RPM as a baseline (in theory) with a direct drive and 15" pitch you obtain 56.8 mph (minus 20% slippage/drag) = 45.5 mph
At 1.5:1 reduction at the same rpm (in theory) you obtain 37.8 mph (minus 20% slippage/drag) = 30 mph.
Therefore, if you happen to have the 1.5 reduction, using a 23" prop (in theory) at the same 4000 rpm would provide a speed of 58 mph (minus the 20% slippage/drag+ = 46 mph
The 20% drag factor should be about right for the v-drive and deep vee hull, but at least I used the same factor for each example. This assumes the motor will have enough power to spin a prop to 4000 rpm too.
So at this point I'm guessing and just like to play around with the math a bit. If you have a 2.0 or 2.5 reduction then that obviously will not work.
Chad and Craig L will undoubtely have some comments on the ratio as well. Give us some more details. This assumes of course, you are going to convert the boat back to an original v-drive system. That, my friend, is going to be a lot of work but of course it CAN be done. You will need a shaft strut at the proper angle to do this, and then will have to bore the hole for the shaft, and will need to install a protrusion in the hull to accept the shaft log.
In the end you will have an original power v-drive boat. The power ratingon the motor may be down from the original specification but if it needs any work at all, it would be quite easy to bump that up. I am not a big fan of the Q motors, they provide good power and they are well engineered, but I would be tempted to convert the motor to an F cofiguration and install a high performance intake manifold like the Edelbrock Performer RPM,
As you can see, once you start getting into all of those issues, the project starts getting more complicated and the cost starts creeping up fast.