That is a nice ride! You can think about lead acid batteries and here's my ideas on it:
Cheep! You can get three sets of lead acid packs for the price of one lithium pack
Easy to charge! They are much more forgiving to use and abuse, especially when they are used off road, with all the bumps and rabbit holes. You also don't need special chargers or expensive battery management systems.
Weight! Normally, we try to get rid of weight as much as possible. Lighter load means further travel. But if you are using it around the yard, you'll never be too far from home. So if the batts die, it's only a short walk
Also, that broken gas engine is a heavy beast. Once that is removed from the frame, 2/3 of the rides total weight is gone. That spells trouble when steep hills, the trailer, or mud comes around. You will need some of that weight back to stabilize the ride, mostly you will need weight on your front wheel to help turn and get out of mud slicks without flipping yourself.
If you wanted to go crazy, instead of one BLDC motor, you should get three Magic Pies. The only issue there is attaching the pie to that tire. But that would be baddass to have three independently motored wheels, you'll never get stuck in mud again! Also that then frees up the entire space where your engine/trans was; now you have even more space for batteries!
What do you think?