i like these trikes, but personally hate the briggs, unreliable in my opinion. i would either look for a honda 9 or better or a harbor freight 13.5 electric start clone. something with lights and accesory power for a winch. and be able to pull whatever trailer without strain. the TAV tahts on there should be fine to handle the motor.
also, with all that space around the rear axle, i would run a higher large axle on a jack shaft, and 1:1 small sprockets between that and the axle. give you more gearing options and higher clearancea at the axle