A lawn mower and generator are completely different applications, most lawn mowers are going to run at around 3000RPM where the generator is going to want to run at 3600RPM, on a flat head you start running into severe air scavenging problems when you get up into the high RPM range. But unless that 3.5hp Briggs was super worn out it's going to make more power and have much better torque than a 79cc OHV engine and overall be a much better mower engine.
That simply is not correct. It doesn't matter what it is attached to, an engine is an engine. I only used my generator as an example because it uses ALL of the horsepower at a known efficiency. The "3.5hp" sticker on the 79cc engine is perfectly accurate, it creates more power at any RPM than the old POS Briggs 3.5hp flathead. The Briggs was rated at 3600 too, even though the lawnmower isn't allowed to go that fast. So you do not get 3.5hp from a 3.5hp Briggs classic on a lawnmower at 3200 RPM.
None of this argument is relevant without a torque curve, which you will never find.
At any given RPM a modern OHV Honda clone creates more power using less fuel, runs cooler, and has cleaner exhaust, with a longer overall life expectancy. This is simply a fact, not an opinion. The old B&S "classic" were full of plastic parts, AWFUL efficiency and poor manufacturing, plastic carburetors, and in general, an awful noisy, outdated engine. That was fine when they first released it in the 1950s - not so much 70+ years later.
It is clear you have no experience with these chinese engines and questionable experience with Briggs, you are simply incorrect. Sorry, facts can be inconvenient.
Don't get me wrong, I used to be a die hard Briggs fan, and even used to be a Briggs and Stratton Master Service Technician, but, there is a reason they went bankrupt. If you are looking for a cheap homeowner grade gas powered anything, China will give you more for your money in 2022, and their engines are actually very good. 25 years ago I'd never say such a thing, the world has changed.
Bottom line - if you're trying to say this 79cc lawnmower is somehow less powerful than the old cheapest model Briggs 3.5hp,
you are simply wrong, and nothing else.