Smart Pie 4 and NMC 24V 20Ah Battery

[trex]

...

Integrated batteries and controllers are great - although that woosh battery looks huge!

You can use any 36V battery with the GSM, the bigger the better.
BTW, the bike on the picture has a very small triangle because of the small frame and rear suspension.
The 11.6AH round bottle is just about small enough to go in, the Oxygen downtube battery is bigger than that and won't fit inside that triangle.
This is a more representative of a crank drive conversion:

In general, crank drive conversions are easier to fit and neater in appearance because controller and pedal rotational sensor are integrated inside. Also, there is no way rain water can get into the electronics.

 

[London Pie]

That looks good - are crank drives okay for gates carbon belts?

 

[trex]

you can get a BCD adapter for the crank drive kits but that would push up the budget. I guess you know this already.