Skyteam have a solution, they are building a replica dax(skymax) with a 5.5 l fuel tank.
The snag, this tank will not fit in a honda, jincheng, zhenhua or older skyteam frame because they made the frame slightly wider under the seat...
However there are 3.5 liter metal tanks out now that can be made to fit into a normal frame(do not know which brand they are though
)
From what i read you have to bend the original tank brackets to get it into place and then it is resting on the frame, not so ideal...