First of all, make sure your carb is a genuine Solex. It should say Solex, Deutsche Vergaser co, Made In Germany on it, or something along those lines. If it says anything else (BACAR, Käfer, Brosol, or any country of origin that's not Germany, or no country of origin at all) throw it in the trash and look for a genuine one.
You have to extract the old bushes, which is reasonably hard if they are bronze. You can try to press or tap them out, one method is to tap a thread in the bush, and then use a long bolt or something, but supporting the carb body during this is always problematic. If you heat the carb body, it helps.
Once you have the bushes out, just measure the resulting bores (as the bronze bushes will no doubt shave some alloy off the sides as they come out, so the diameters could vary), and make a bronze bush to fit in the bore, with a hole in the center roughly the size of the spindle (slightly less). Make the bushes o/d a little bit bigger then the bores, for an interference fit, but only slightly, since those alloy carb castings are really really weak. Do each side separately, don't just measure one side and make two bushes.
Now press the bushes into the casing.
Once they are in, make sure your throttle spindle is not oval, and get an adjustable reamer and ream the insides of the bushes (together, at the same time, using the same reamer through both, to keep it parallel). Take small cuts until you can fit the spindle in using moderate hand-pressure, and you can turn the spindle once it's in position with two fingers, without too much effort. If it has a little preload, its not a problem, as long as you can comfortably turn it with two fingers, stop there. Since on the beetle engine the throttle return spring is quiet powerful, a little too tight is better then a little too loose (since if it's too loose you have to start again).
That's how I do it anyway. I gave up doing it with set measurements ages ago, because there are variations in carbs. I just do each one bespoke, like I described.