Swedish 50:s motorbike with 3 kw hub motor - battery suggestions?

Hi
I´m currently planning the conversion of a Swedish motorbike from the 50:s. The plan is to use a hub motor kit and using the frame space for battery placement.
The bike is a hardtail and probably had a top speed of 60-70 km/h to begin with, so I think it is a resonable goal for the conversion as well. (The original engine was 4 hp 2 stroke).

My plan is to use a kit like this:

What would you recomend when it comes to batteries? A range of 40-50 km would be a good start.

For 70kmh you need around 5kw motor power.
So with72V 50Ah 3,5kwh you can drive 50km with "full" speed. If you drive slower, you need less kwh.

It is easier to build with a hub motor and battery voltage is about the max speed you can get in a normal setup so aim for that.

My suggestion would also be a 72v battery and about 50ah as the former post states. That will give you a max speed of about 70 km / h depending on your controller choice. (My 65ah battery of the same voltage gives me around 80km of distance with an energy consumption of 45w/km. Slower speed gives more range and if your speeds stay at or below 50km/h you can build a smaller battery in the 35-40ah range)

Are you looking to keep the original look of the bike or what is your design plan?

Thanks for the replies! Now i have a good starting point for the battery dimensions. Designvise I will be somewhat of a modified/customised originial. The bike need a bit of work itself, but I do want to start with getting the big picture and components into place before getting to the work on fenders and similar.