I wish someone would make one of those tanks with an offset filler. I appreciate the look of motorized bikes as much as the next guy, just don’t care for the top mount tanks, even on a Whizzer they detract from the lines of the bike, IMO.
It might have (I sold it long ago,) but the cap wouldn't come off properly and it would have been impossible to fill up.
There are folks who build custom in-frame tanks. I have used them in my two "keepers."