You do not give enough information to enable one to be very helpful to you. What are your requirements for you off road rover? You may look at similar devices (like electric scooters perhaps) via the internet and see what size motors they use.
Here are some 12 volt motors from 1hp to 1/3 hp.
https://www.ebay.com/itm/233312841977?_ if that link does not work search on ebay for "DC MOTOR, 1/3~3HP, 56C, 12/24/90/180V, 1750RPM, TEFC, Permanent Magnet"