I reinforce the weakness in the arms of the old RP casting of the Stone Rhino by drilling all the way down the arm with a 1mm drill bit until it's about to come out of the underside of the gun housing, and then supergluing in a 1mm steel rod. I bend the rod before I put it in so that I can then superglue the other end into the main body of the mech, and you end up with a real sold mech. There is still a little weakness in the overhead gun because it's so thin. I have tried coring that with steel rod, like you do with the old shadowhawk gun, but it's a real pain to get right.
Gotta say I love the old RP Behemoth. Never even built up one of the new ones, but the piccies make them look bad.
I just buy them when they turn up on eBay. Even if they're broke,so long as all the buts are there, they can ve fixed up like new.