To continue, the install is not that difficult, but not without issues. Getting a standard Jabsco marine head will prove a simple intall on its own. How difficult the job then becomes depends on your choice of tank size. Bear in mind that the original tank was a custom job designed to fit the space. It was sized to be dropped into the v-berth through the cutout made by the manufacturer in the top of the v-berth, and had a multi-faceted bottom shaped to fit securely against the severe v-bottom of that area of the hull.
I found it not possible to get an exact fit, and I wanted greater capacity. I needed to cut a larger access hole, and then for the install I covered the area of the hull where the tank would rest with wax paper and the tank with tinfoil. This allowed the positioning of the tank and the connection of the hoses and then to expanding-foam the voids between the new tank and the hull to ensure that the tank would be fully supported as the contents will weigh a lot when near full.
It was a bear to attach the sanitary hose, as these are a very tight fit. My solution was to keep handy boiling water in which I could emerse the hose end to heat and soften prior to forcing on to the tank fitting.
Do you need to haul out to do this job? If the existing thru hulls have working valves, then the raw water input hose can be connected to one of these and then to the head input. As to the rest, all othe rholes are above the waterline and hence no haulout should be required.