yes, engine has to come out, ebox out (I bought my tank from tonka as well)
shove it in the hull, sit on the grab bar and kick it into the front area with your feet, its a tight squeeze.
cut down the storage bin, re-route the fuel filler (a little bit of creativity involved)
I can't remember if the ebox had to be moved back a little bit or not, I remember it (bracket included) coming out and it being re-riveted, I just can't recall if it had to be moved when putting it back in.
the big advantage with those extra gallons (9 to 12 right?) was the extra few miles of range that it added, when I was going on rides with the 9 gallon tank I pretty much had a MAX 45 mile range, this squeezed it up enough that my fellow riders didn't groan when I brought the XP along that day.