I'm not very familiar with how the retro goes, but if you do a search in the 3rd gen section, you'll find quite a few discussions on people who have switched out the tank for a Ford (I believe) tank that offers a bit more capacity. I'm pretty sure i've seen others talk about fuel cells also.
Just go here, search for gas or fuel or whatever, and search just thread titles and you'll probably find quite a few of them.