Lets see...
The smaller packaging is way less efficient, with wasted volume on the ends of the headways, etc. But:
Approx U-1
Length : 7.8 ins (200mm)
Width : 5.2" (135mm)
Height : 6.3" (160mm)
???
Or whatever battery compartment allows.
Then a 10Ah headway is 135mm including screw tab, 40mm x 40mm with building block.
So width - same or a fraction more with connections etc. May fit, would need to see/measure battery box.
4 high 160mm
5 long is 200mm
So 2x 20 cells is possible I think. So 40 cells. In 5 cell groups, to give 5p 8s. Or a 50Ah battery in place of a 35Ah lead U-1 system.
But at high rate the usable lead Ah is almost halved. Lithium is unchanged. No peukert. So range would be improved by approx 2.5 times and it would be much lower resistance, and last around 10 to 15 years.
The ONLY dificulty is the length of the cells, with srews and connectors will be about 3 to 5mm more than the U1 batteries. This may not matter, I cant see the battery box. Make 2 cardboard boxes, 140mm x 200 x 160 and see if they will fit. If so order today!
www.evassemble.com