worksman, torker and husky all make heavy duty "unisex" bikes, but i think the problem with making a boardtracker with girls bike, is without a top tube, she'd be laying on the engine if you had the low-slung theme going.
if i were doing it, i'd find an old vintage 24" boys cruiser. they're just a little smaller, and if you looped out the frame, the motor should fit right in there.