For an in-frame battery mount a "halong" style might fit, but if it didn't you could mount it under the down-tube. There may be water bottle rivets already there or they could be installed. This is not a cheap battery, but if want to match the quality of the bike, this is the place. Battery diamensions show here;
https://em3ev.com/shop/52v-14s4p-shark/
Since you would be buying a premium battery, you might as well go with a 52 Volt system. This is as high as one can go with standard 48 V controllers. I would mate it with medium size rear hub motor (around 3 to 5 Kg.s, most likely a Bafang) that has a motor speed-range of 250 to 270 rpm @ 36V for a top speed of 25 to 27 mph.
To go faster, you need more battery you don't really have space for.
This would ok for lite trail riding, but it won't do steep hills, again a larger batt/motor would be required.
But this set-up would be a great low-maintance urban cruiser.
Range would be about 20 miles, which is further than one would think.