There are some "fitted" replacements for the "lower portion" but you can also just use any old deck mount on the floor under where the old one was.
You will need to do the "jumper trick" with the old 6300 fuse panel or replace it with a 7300 style fuse panel which ever route you go. (don't get a 7300 kit!!!--still only a one stage converter at 13.8v) There is an Ultra kit but that is a WFCO, so stay away!!!
That leaves PowerMax, Iota, and PDs. there are pros and cons to each of those, so to pick the right one for you, you need to know what charging profile you want to go with your camping style, available gen time, etc.
Here is one example from Randy's site (use the hornerfamily link)
http://www.bestconverter.com/Parallax-to-Boondocker-4-Stage_ep_24-1.htmlOR you can just save a lot of work and leave the 6300 there and put any deck mount you like somewhere else and use that as your new converter.