I've done this conversion. It works fine. The belly-sac does not get in the way, provided you don't go crazy on decorating the base. I magnetised it with just a pair of magnets glued into a glob of greenstuff either side, with the termagant bits attached to one side (I chose the left). The arms are straightforward magnetising, except you might want slightly stronger magnets than for, say, a carnifex as some are quite heavy.
Regarding the legs, the rear legs are stong enough to support the whole thing. I built mine with the scything talons detachable, and one front set of claws glued, as both the tervigon and tyrannofex use them, but the back legs will hold it on their own. If you are gluing the front claws, just be careful to make them wide enough to remove your magnetised belly-sac.
So the whole thing is fully interchangeable with one set of arm sockets, about 4 pairs of arms, the belly, and the two heads.
Just finished painting mine, in time to take it to the 40konline tournament.