Okay, this is a challenge!
On one level, I think this is pretty straight forward. The Kickstarter Timber Wolf is the place to start; as plastic, it's a heck of a lot easier to mod. Remove the missile launchers (retain for future mods, or send to me ;) and trim off the gun pods at the 'wrist'.
For the left arm, I'd recommend the right arm from the IWM Scourge. Remove the power cable, and plug onto the left wrist. For the right arm, the right arm from the Avatar looks closest to the mini; however, for HLL and HML, you could at a pinch leave the existing right arm as is. Pinning recommended. If any of that doesn't make sense, PM me, I can wax prolific ;)
The only thing is that the Mad Cat II is a 90-tonner, while the Timber Wolf is 75 tons. The KS Timber Wolf is plenty bulky as is, but I'd probably cheat - separate the feet from the black hex base, carefully; build up the hex base by 1/4"or more with putty/clay/etc, then glue back in place, for some spurious extra size.
Hope that helps some!