Re: Need ideas for a new fuel tank......
Although I don't have that engine, you may want to check out some lawn mower shops for gas tanks....Most Horizontal engines have a gas tank in a similar location to that engine but you may have to fab up a mounting plate or something since I kinda doubt a tank from another application would be a direct bolt up...
I have a 13 HP Honda (I think a GX 360?) on a power washer that has a huge gas tank (I think the tank mounts are pretty custom to that engine tho so a custom made adapter plate is a given needed on this...On the other hand ....My snow blower (Techumsh powered) has a pretty good sized tank too and it is mounted off a metal plate that could be easy to drill out (the mounting plate that is) for making different mounting holes...This tank and factory mount would probably make this easier to fit onto different engines.
As another possible idea.... how about a second aux tank or simply an additional fuel container you can carry (I've heard of folks using aluminum containers and using bicycle water bottle mounts to carry the extra fuel) you can refuel when needed.
Hope this helps you.