Without changing ANYTHING? Exactly what you have.
A predator 301 would probably fit, but you'd have to change the driver sheave to 1" ID, and you would be pushing the limits of the TAV2, and might have to modify the rear oil port to make it fit.
A 420 might also fit, particularly with a remote mounted fuel tank, but you'd have to upgrade to a 40 series torque converter to handle the power, and a 40 series on a backplate is not an ideal setup. Running an independent jackshaft would be better but would require modification.
If you look for used engines, you might find a Tecumseh or Briggs in the 8-10 hp range with a 3/4" shaft.
And any larger engine may require modification of the engine mounting plate to accommodate a different bolt pattern.