Well this is going to be anything but stock, I’m putting a 25hp diesel to it. From prior research I found that these units are able to handle about 30hp so I'm pushing it as it is. But the biggest killer from what I can tell is going to be heat breaking down the fluid, so I’m thinking of adding a cooler to help the drive shaft fan.
Not to worry.
The hydro may be able to handle 30 hp, but the rear tires will run out of traction at about 6 hp unless you have a huge amount of weight. My MF1655 carries a minimum of 400 lb of rear end ballast and I can add over 450 more and it will still spin the chained tires on dirt at 2/3 throttle with a 16 hp Onan. It hasn't even had the drive shaft fan for cooling since 1996.
About the only real benefit that higher horsepower has with a hydro is faster acceleration. It won't deliver any more torque to the rear axle than a smaller engine, but it will deliver more horsepower over a broader range such as climbing a slope while towing a heavy trailer.
Entry level LT's often have a Tuff Torq K46 hydro that is rated for 2.75 hp. They come with engines with as much as 20 hp and they run out of traction before they run out of even that small amount of hydro horsepower.
High horsepower is for the powered implements, not for the drive of GT's.
Cooling is required for sustained operations close to traction limits, such as ploughing a field with a triple 12" bottom plough. Considering that most other tasks won't run the hydro temperature over 140° and the high temperature rating is over 190°, I wouldn't get too concerned. The recommended temperature range is 140-160°.