Well that depends on how close you want him to be to the original model.
There are many possible ways to go. I for myself would try to mirror the wargear as closely as possible which means a true combi bolter and no flamer. Just cut up two bolter or bolt-pistols and put them on the back of the claw.
Another possible idea is to use the servo arms as counts-as power claw and use a regular clawless bionic arm instead of the power claw arm. Then you could either put the weapon in his hand or build them into the servo arms. In the second case you could mount two seperate bolters on two arms (a combi bolter is in essence just two bolters strapped together).
Another conversion could be to use exhaust pipes in place of abaddons trophy racks. Its iron warrior'y and creates a similar footprint.
But whatever you do remember that at the end of the day the rule of cool beats all other rules