That seems to be a nice bike
The SLA's are going to be the heaviest part, there will be two 12V to handle the 24Vdc.
Only way to extend distances would be to have more Ahrs available.
Couple ways to do that, add more batteries OR swap these out for two higher Ahr batteries.
My best guess will be they are 10Ahr batts. You could go the expensive way and buy them direct OR go to a battery reseller of your choice, (I use Batteries plus).
OR go different ways, such has buy Li-based battery packs, while expensive will cut down on weight and improve distances.
There is also the DIY way of re-purposing NiCd/NiMh as battery packs, but then your "fuel" gauge will not be accurate due to Ni-based unit having different discharge curves.
Hope this helps, let us know how you make out.
Cheers;
Bruce S