Stan,
Remember these engines were designed and built to a specification at that time.
Common parts were obviously a design criteria (or is it -um) and so was the engine speed.
Now if you wanted to increase the loading on a splash-lubed big end bearing and keep longevity, you would be out of luck with the bearing materials and sizes they were working with. So it was simple physics or basic design calculations to work out the bearing loadings at given rpm.
If that size engine had been designed for 1500rpm the power could have been x2 1/2 and the loading would have been approx the same but the rod lube would not have been ideal and the flywheels would have needed a different design. They could, of course, have used 3 inch crank bearings etc etc etc... but they didn't. So we are left to use the engine within the design parameters distributed by the experts - the designers, builders and suppliers (Listers). Piston wear at 1500rpm would much exceed their longevity calculations, of course! They simply got everything designed right. They did their job and made a good reliable long-lived simple engine which could be maintained adequately even by the owners of the day.
Soooo, the engine was rated for 650 rpm and 6 HP eventually, after they found it was not stressed at 5HP. The change-over-valve (COV) was simply a means to get the engine started in cold conditions, but it was OK to leave it 'engaged' if bearing loading was kept within design limits (ie lower power limit) and that option had the benefits descibed earlier. You are simply not able to keep the bearing loadings within the limits of the design if operating at high power with the COV is screwed in.
As Peter points out an aluminium piston was used later - this would allow higher piston speeds and more power for a given bearing loading.
Bearing materials have improved since 1930, as have oils and fuels. Most engines now produced are high revving and of relatively short life span. They are designed for a life-time cost better than this old Lister design. But they may not be able to cope with waste oil and lard as fuels like these old engine designs. Modern engines are designed 'just' to hold together, by computers. Any surplus strength or metal is designed out. Competing designs have to be competitive and price + lifetime running costs in the 'write-down' period are the accountants' criteria, not how long it might last.
Regards, RAB