Diablo, you said you have used a 2" pulley, compared with the 2.5" which is standard and the 2.75" it had when you bought it. You reported a ground speed of 4.6 km/h, or 2.9 mph. For what it is worth, the average "standard" walking speed for unencumbered male humans is said to be 3 mph, and some crude measurements I have made support this for sustained walking (say, at least half an hour without stopping). However on short walks it may be a bit higher. A lawnmower is a slightly different situation because sometimes you have to engage in tricky steering, which requires a lower speed. Hence my wild guess is that you may have a rather practical general-purpose speed there, but if you were mowing an oval you'd want more, because it is all straight runs and you would end up using it as a physical training exercise to stave off the boredom. (Hence your WACA groundsman's 2.75" pulley, which probably gave him a ground speed of 3.9 mph.)
Getting away from this subjective stuff to something more meaningful, the small horizontal crankshaft side valve Briggs engines like yours are normally governed to 3,600 rpm maximum. The vertical crankshaft ones are traditionally (well, since 1980 anyway - it was higher before that) 3,000 rpm because of blade speed regulations in the US.
