Is the wally world bike the Point Beach?
Walmart.com: 26" Men's Schwinn Point Beach Cruiser Bike: Bikes, Scooters & Skates
I have built several alum. framed motorized bicycles with no problems. Steel is preferred by most though.
A coaster brake alone is a bad idea even on a bicycle not motorized IMHO, doubly so on a MB. At the very least you would want to add front brakes. Gears really come in handy. Helping with hills, starting off and some of us forget no matter how good on gas, they do go dry. Then have to peddle home or to the gas station.