hi all,
i agree with grumpy that if you can find a mill and have it precision cut it would be a pretty permanent repair. even if you have to take it to a shop its only a 5 minute job, probably cost $50. then to enlarge the hub to fit the wider key a file could be used fairly easily and accurately.
as to the drilled pin idea you would need to use a very hight tensile material for the pin,at least 1/4 inch thick stainless to sustain the pressures of the sharp change in motion (theres that bloody inertia again). so if you drilled it as close to the start of the taper you may be OK for the short / medium term, but in the end i would reckon that the high tensile pin needed would strip the alloy rim and mild steed axle causing real problems.
alternatively try mower shops for second hand bits. if you live near hastings, try hastings mowers as they have quite a bit of junk laying around.
welcom to the forum!
regards jay