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Mithril, as sailingdog says, a stub shaft is really an extension of the main camshaft. It will NOT be cheap to have a machine shop fabricate that. As an alternative, you might (if space allows) remove the flywheel and have a machine shop fabricate a new flywheel with the stub shaft attached to that...still ain't gonna be cheap.
But I think Paul is more on the mark with his comments that adding a larger alternator on a single v-belt is just the wrong way to go. Once you hit 80-100 watts, an alternator v-belt slips so much that it is counterproductive. And using two belts (the old way to beat this) was an exercise in futility because unless you kept them matched, one always stretched more than the other, the load became uneven, and you're replacing belts again.
The new and more reliable way is to run the ribbed (serpentine style) belts for higher loads but that still doesn't help you with the shaft side load.
For that, the simplest solution is to use two smaller alternators, installed so they oppose each other and cancel out the side loads from each other. You may still need a custom double-pulley to make them fit, or some other machine work. Pulleys and shaft loads are all basic work for a machine shop, if you can take some pictures of the business end of your engine and stop by a machine shop in your area when things aren't frantic (not 8AM, not 4PM ) they'll often be able to give you some ideas. They may be able to install an idler pulley on the opposite side from the (one) alternator, to take up some of the side load.
But if the machine work gets too expensive...sometimes a genset makes a more effective solution. Or a wind generator, or solar.