Will the lawnmower blade be powered from the same battery? If so, it will probably take a lot more power to turn the blade than to move the chassis. That will be a big factor in choosing the correct battery.
There doesn’t seem to be any information about current consumption. Normally you would choose a battery at or close to the motor’s rated voltage, and the capacity is determined based on the average current consumption of the motors. Take a look at the “Capacity” section here:
I found conflicting information about how much current a Jazzy wheelchair consumes. You might want to measure it for your application, since the overall weight of the mower will be lower than if a human adult was sitting on it. “Guesstimate” might be around 8A per motor, so quick math, factoring in some losses, might be a 24V lead acid or 22.2V LiPo, 20Ah battery pack.
What I would recommend is to assemble a frame with all the components such as the drive motors you are using and with the lawnmower engine attached. Then find a battery of voltage similar to what you will use, 12 or 24V, to use temporarily. A car battery or two, or the wheelchair batteries, should do fine. Then run it and measure the current.
In fact, since you are using wheelchair motors, with weight and usage similar to the original wheelchair use, the easiest route is probably just to use the same type of battery as the wheelchair used. That’s what I would do.