Many people, myself included, tend to think of the new Haruspex as a pile of shite. Thats not to say they don't like the model. They would love to build and paint the kit as a Haruspex but prefer the rules for the Exocrine. Fortunately, GW built the model so that it is very easy to swap the heads and arms and get the best of both worlds. You can now enjoy painting your Haruspex and scaring the cat with it, but use it as an Exocrine when you fancy a game of 40k. I was able to magnetise both variants using 9 magnets in total.
You only need one pair of magnets for the head (as above). Both head options dovetail perfectly into the body, almost supporting their own weight (you can't fault the quality of GW plastic models. Just the rules. Oh, and finecast. My new venomthrope was worse than the straw man in Wizard of Oz and needed some serious (green) stuffing)
I use ModiFX 4.75mm Rare Earth Magnets because, well, the shop doesn't sell anything else. You get a drill bit in the starter set which is the same diameter as the magnets. This is invaluable. I just manually drill holes in styrene because its soft.
The corresponding magnets on the head options are shown here. The Haruspex is easy and a magnet nicely fits into the notch on the back of his head. To get a magnet in exactly the same place on the Exocrine head however is a bit trickier and I had to glue a piece of sprue inside to hold it.
A) I covered the magnet on the body with some blu-tac and put a blob of paint on it. B) I fit the head onto the body and the paint marked exactly where the magnet should go. C) I had misjudged the position, so I had to glue a bit of extra sprue on to fit the entire magnet. Hmm, maybe I'm not actually qualified to show other people how to magnetise things. D) Note that the magnet isn't flush with the gun. I had to make it stick out a bit to get contact with the other magnet. You can tell you've done it properly when you can hear the magnets clicking together.
Fitting magnets on the arms is easy. Note that the Crushing Claws are hollow so you'll probably have to drill right through, leaving a void. This is fine if you've used the drill bit that came with the set and the magnet will fit snugly. If you bugger it up don't worry. Squeeze some green/grey stuff into the hole, press the magnet into place and do a few test fittings as it slowly dries to get the pose you want.
You can now field an Exocrine in your games but still play with your Haruspex to your heart's content. Don't do it too much though, or you'll go blind.