With small tractors such as these it really ain't so much about the horsepower, it's about traction first and gearing second. If you spin a wheel pulling a heavy load then your tractor already has more HP and torque then it can put on the ground... you have to find a way to put that wasted power on the ground... and adding more HP won't help at all. If your tractor can't get the load moving, pull it up hill, or over rough ground without smoking the belt and killing the engine you need lower gearing... more HP will just chew up the belt faster.
Back in the day they made two types of "yard" tractors...
One was called a "Lawn Tractor", usually a bit lighter built and higher geared, cost less money and mostly intended for mowing yards. There may have been a limited selection of attachments, such as a snow blower for doing the driveway.
The second type was called a "Garden Tractor", they were built heavier and cost more money. Usually there was a long list of attachments... just about anything available for full size tractors, including loaders, plows and whatnot. All these attachments needed power to work, and by supplying lower gearing the operator could balance the percentage of power required by the drive wheels... leaving more power for the attachment. It also allowed the tractor move slowly at full throttle when required, such as using the garden tiller attachment... using full throttle to power the tiller but in low gear the tracker moved at a snails pace, leaving near 100% of the power for the tiller. The low gearing was often a real transmission (instead of some slip-pulley design) with more gears (maybe 7 or 8 instead of 3 or 4), or like my old Sears Custom XL, it had a two-speed axle that allowed super-low gearing throughout the full range of the transmission, including reverse.
Either way, adding a couple more HP to your tractor won't make enough difference to justify the expense and effort.