I saw on ebay from Germany not so long ago but can't find it again now, an early SFC GRP tank that had a steel inner tank fitted to retain the original appearance while safer for racing.
I too would be interested in an inner tank to fit closely inside the American Eagle sport tank - any others if we found somebody to build some at a sensible price?
The original is a fairly squat profile, not so tall as either SF tank or the GT so can't afford to waste much inside space inside a top shell but a liner tank could be pretty basic with a couple of gallons capacity, fit the front and rear mount positions and inside the upper cover from the original aligning the filler. The steel base of SF tank could be used as a pattern.
Any wider interest?