Thanks for that smythie, I thought that carburetor was introduced a bit later than 30 years ago (1980). If you can get more power at high speed by adjusting the idle jet, it is obviously running lean for some reason. (The idle jet shouldn't even be emitting a significant amount of fuel at high speed: it should only produce a flow when the throttle butterfly is in the nearly-closed position). You can check whether the engine is running lean by looking at the spark plug colour, and by slightly applying the choke and seeing whether the power increases.
There are engines that give better fuel consumption than your old Briggs, but they tend to be rather more complicated and perhaps to give a bit more trouble (the OHV Briggs V-twins are an example). For a contractor who has his mower professionally serviced, a modern engine of a reputable brand is probably the best way to go - the fuel saving and quieter, faster mowing can be important. The Loncin engine is of a somewhat more modern design than yours, but that wouldn't console you much if it gave trouble and/or you couldn't get the right parts and service information, quickly, when you needed them.
