My own personal opinion would be to stick with the Irvine - especially if you intend to do OMRA / BMPRS style racing. Two main reasons:
1) If you put a CMB 67 in it, and get the CMB performing as it should, I believe that you will have difficulty controlling it amongst other boats - on a course with tight turns, especially on rough water.
2) If you get a CMB 67 running in its power band, you will need something like (probably at least), 3L of fuel for a 1/2 hour race. Ballpark tank sizes for CMBs are around 1.5L for a .21, 2.5L for a .45, and about a gallon for a .90. My own CMB .45 won't do 1/2 Hour with 2L. I think that accommodating that much fuel in a Fantome will be difficult, and the size of the fuel tank(s) will end up determining the layout of the other components. Whereas, as mentioned earlier, you should be able to squeeze 1/2 hour out of 1L with the Irvine.
Realistically, a Fantome is unlikely to be one of the fastest "B" class boats around the course. That doesn't mean that you can't race it, or that you shouldn't race it. It just means that you need to focus on reliability, and keeping it on the water. A reasonable pace, with a non-stop run, will get much better results / placing, than a very fast boat with lots of stops.
One other thought, not sure where the prop is / was on that boat, but in keeping with the full sized versions, the model Faireys often have the prop very close to the rudder. This is not really good for handling, the prop blast acts on the rudder, and tends to cause nasties. If you are rejigging the shaft anyway, and if it is practical (within the confines of where the engine needs to go, and a sensible shaft angle), I would try to get somewhere around 3" between prop and rudder.
Just my 2p worth............