My install was not to replace the wallas, dont have one. Its a simple lay out really. You have the main parts. Heater,, tank, thermostat, combustion air intake, burned air exhaust, cool air intake and heated air vents. The big challenge is figuring where you are going to put everything. Think about it and then think about it again. Can you get to it easily? can you get to the tank and fill it or remove it to fill. Can I reach the control unit from the wheel? from the berth when whinny wife said its to cold to hot? Where is the fuel pump and can you get to it? if it leaks what will get fuel on it? will the exhaust burn the dock or my trailer? will the exhaust port hit the trailer guides? or will it burn the lines or fenders? Just sit in the boat and think about it.
Ok here are my thoughts on the air flow. The install instructions ( written for a truck or bus) call for the air, both combustion and heated, to be brought from the out side. I choose to have them both brought from the inside. Lot less trouble for the install and I believe that it keeps the cabin drier by pulling all the moisture out of the boat and either heats it or burns it off. The heated air pick up is close to the floor pulling all the heavy cold air off the floor. My cool air combustion intake is also on the floor but behind a cabinet. You can feel the air and the cabin getting drier when the heat is one. I saw no reason to drill two more holes in the side of the boat just for these air intakes. I only have the one thru hull for the combustion exhaust.
Don't waste money on a fuel tank from sure or scan marine. I just modified a cheap red 5 gallon (?) plastic gas can I had in the garage. Why? cause I am cheap that's why. And is way fun and easy. All the parts where on the kit or on the can I just swapped out the parts I needed from each to get what I wanted. works great.
The one thing I did not install ,but will be doing this winter, is a water drain for the exhaust line. In really bad weather you can get water in this line from a large wave. I have a U in the line but no drain. water cant get tot the unit but if water is in the exhaust the unit cant create enough pressure to push the water out and it will not light.
I hope this helps you when it comes time to install your unit.