Ahh, very inspiring... (I've got a spare Agility available for all kinds of experiments
)
Brilliant idea, recycling the backrest as a seat !!
Not so sure about simply lengthening the frame though, the extra pipe may be strong enough for itself, but the extra length of the resulting "flat" frame puts a higher load on the existing pieces of tube near the engine mounts and the front.
I think it would be wise to weld an extra tube under the frame, positioned lower than the side tubes and perhaps with some cross-connection to the side tubes, to keep the frame from bending vertically and from eventually breaking the tubes near the engine mounts or front because of metal fatigue....
Keep those pictures coming !