A 1000 Watt gear motor along with a top speed gearing of around 3 MPH is needed for this type of wagon. Here is the calculation:
This calculator can be found on our Motor Power Calculator page.
Our kit item # KIT-481000G-20 would be suitable for this project:
The 48V 50Ah battery pack in KIT-481000G-20 is estimated to provide a 153 minute runtime on flat ground with some hills which sounds like more than you need. A 48 Volt 35Ah battery pack is estimated to provide a 100 minute runtime on flat ground with some hills which sounds closer to what you need. These runtime calculations are made on our battery pack ride time calculator on this page: https://electricscooterparts.com/battery-pack-calculator.html
If you are interested in KIT-481000G-20 however with a 35Ah battery pack then please create a support ticket and request new version of the KIT-481000G-20 with a 35Ah battery pack.