The MH in sig had a 6345. I removed the "lower portion" (the converter), jumpered the fuse panel so it was not "split" (the 6300 converters have a red and a blue pos wire where newer converters have just the one pos wire that does all the fuses.
You would need a new fuse panel unless you jumpered the red and blue on the old one. I then put in a straight deck mount converter handy to the battery bank (I chose a PowerMax)
You don't have to get a new replacement "lower portion", but you can. A deck mount will fit there or go to a better location nearer to the batteries. As mentioned above by others, PD makes replacement lower portions and so does PowerMax and even WFCO, but don't get a WFCO! (the Ultra kit--frying pan into the fire deal) and don't get a 7300 either.
You can get any "three-stage" brand deck mount 45 amper and it will be good enough.