Yeah its fairly easy to magnetise the whole
sh'bang witht he predator
for the turret I put a magnet each side of the main turret piece (where the pegs used to fit into) and witht he guns i clipped the pegs off and drilled holes either side for 2 more magnets. Probably using more than is needed there but it retains the up/down posability, provided your magnet fit is decent then it doesnt droop down too
The sponson guns were easy to swap with plain plates with a magnet in the recess and one onthe back of the plate, didnt even need to drill that one.
the guns to be changable on the sponson mounts was the most tricky part for me, In the end i put a single magnet inside the guns before glueing the two halfs together, and then mounted one into the tiny little rod that the guns mount to, that one was smaller than the res at 2mm diameter by 1mm depth, all the others i used were 3mm diameter by 1.5mm depth.
I even managed to have the hunter killer mount magnetised, a 2mm one in the center of the cross that is its mounting bracket, and one fitted flush intot he armour plate where you choose with milliput filling and sanding job to hide it

is well enough hidden once painted up if you sand it well enough.
edit; also be sure to make the ammo/battery box for the back of the turret changabel too so you can have lascannons with batteries and auto cannons with ammo boxes