Igor, manufacturers of most everyday engines would like to try to achieve a flat torque curve (the same torque at all engine speeds) because this gives the most flexible engine response, and very high power at high speeds. In practice you can't though, unless you cheat (turbocharge, and use a controlled waste-gate to control the intake pressure to give the torque curve you want). This chart shows the power and torque curves for the Honda GV100, a 97 cc side valve engine with a compression ratio of 5.6 to one:

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As you can see, it has maximum torque at 2,900 RPM and over 80% of maximum torque throughout the published speed range. You can also see that at the maximum governed speed of equivalent (side valve, 6:1 compression) B&S vertical crankshaft engines - that is, 3,000 rpm - it has less than 1.6 "recommended" horsepower. Don't you like Honda's way of hinting that the "maximum" (i.e. gross) horsepower line might not actually be achievable in practice for an installed engine?

With regard to your 45 and 625 B&S engines, you had proved that the 45 had insufficient performance, so a 39% increase in power and torque was bound to be beneficial. The actual power of the engines is less important than whether or not they can do the job you need to do.

With regard to the OHV and OHC mower engines, in practice they normally have much higher compression ratios than the side valve ones (8:1 instead of 6:1). This results in a need for different design criteria for camshaft, ports and valves, all aimed at getting more useful power at maximum governed speed combined with lower fuel consumption, compared with a side valve engine. One reason contractors just about always use OHV or OHC mowers is the much, much lower fuel consumption. The offsetting disadvantage is greater cost and complexity, plus a need for more precision in service operations.