How in HECK to you figure HP on a B&S engine?

Yeah well;
(Torque x RPM)/5250 still equals hp,
so I never saw the problem; just crank up the stinking governor,
I mean; 8.75 ftpounds,
at 3000 is 5hp,
at 3300 is 5.5hp,
at 3600 is 6hp
and if that don't do it, install a bigger carb and/or a decent big-can muffler with a clean spark arrestor, and/or a torque-tube; and when all else fails, shave the head and/or retime the cam, and add some tuning.

The same HotRod tricks that turn your pos low-compression 318. into a fire-breathing Monster from hell, also work on your lawnmower; even the lowest of the lowly, the 2-cycle LawnBoy
It's can all be done with the same long-block, just revved higher.
It's the exact same game that Chrysler played back in the day....

And honestly, when it comes to power-equipment, who cares which number gets quoted? In the auto-industry, the small engines went obsolete for a reason.
And, just like in HotRods;
Just buy the right sized engine for the job, and
bigger is always better.
And just like 318 hotrodders find out, more revs equals more action.
And besides, walk-behind lawnmowers, almost never run at rated rpm , and rarely at WOT, mostly cuz the governors are handicapped right from the showroom floor, plus the carbs have over the years been recalibrated leaner every year.

I just go to the landfill on any Saturday, and collect ten-year old machines, as they come in thru the gate. For the price of a carb-kit and a couple of hours work, you get a powerhouse that starts on the first pull, and is ready to race in seconds. Before Covid I had a nice little side-business refurbishing that older stuff.
I mean, on push-mowers, they just need a tune-up, and a blade sharpening, and the odd one needs a flywheel key, or a cable, or maybe a wheel. I mean unless there's an exit hole in the side of the block......
But-um, if you buy a brand new string-trimmer, yur gonna have to buy some tiny jet-drills, or you are gonna be one unhappy yardman. It may take you three or more tries to get it rich enough to burn 87E10. and then you still gotta deal with the low-speed circuit. and the transition..........
But after a couple of hours in the shop, you'll have a nice reliable machine that starts, runs, and cuts weeds, like it was originally created to do.