The bike is really odd, my dad bought a pair of them for a very reasonable price a very long time ago, but never found a use for them. They used these old UPS batteries, sealed lead acid. The batteries never lasted that long for various reasons and weighed a ton.
The original batteries were 2x 12VDC lead acid batteries. My dad was under the impression that the bike was identical to a 36VDC version sold in other jurisdictions.
So I bought a 36VDC lithium ion battery that claims 7.8 Amp-Hours. That would work out to about 280 Watt-hours. This is comparable on paper to the 288 Watt-hours the original batteries had, but there's definitely more to the picture.
I bought the batteries in the middle of the end of the world, so I limited my search at the time to batteries that were available in my own country. If I were to do the same project today, there are batteries that are significantly more energy dense for not a lot more money.
I've run my initial test run and it works! One thing I immediately noticed is that I lay on the gas too much, it cuts out the electronics. I have a feeling the higher voltage means much more current is available and the circuit is cutting itself out temporarily to protect the motor and electronics.
The biggest things I did for this project was designing and printing the cover for the battery and setting up the electrical connections. I wanted to keep the battery removable because every year the batteries in the bike die over the winter. As it is, the connector is a standard plug I can remove, and it slides into the original battery holder guide.
There's one or two little things I want to change in the next little while. The battery sits on the bottom of the holder. There's nothing wrong with that per-se, but I feel like over hours of use it'd be a potential point of failure, so I'm going to design a small stopper that fits in the holder slot so the weight of the battery is held by the holder rather than rubbing on the bottom.