Just played around a bit and I think it will be really tight with a stickpack.
Maybe if I took off the end caps and resolderd the cables...

On the other hand a new motor + packs would easily get to 300€. I would rather use the 12 cell setup until the cells die and then buy lithium and a new motor.

Another option would be to go with less speed and put a 9L or something like that in there.
Do you have an Idea how much speed I would lose with a motor that's appropriate for 12 cells?