My ALDENS Magic ZZ machine ( looks like a Necchi BU ).
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It had a motor bracket that had no adjustments. It forced you to use a rubber band belt. I never could accept that so I set out to either replace the motor bracket with one that could be adjusted, or to modify the existing one.
The existing one, had the typical slot for the usual single screw, then an additional hole above the slot for one more screw. No adjustment possible. The machine does not have a boss like normal machines so you have to use two screws to hold the bracket straight. You can just barely see this in the pic below.
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At first I thought this can't be the original motor and bracket then I saw a pic of another machine just like this one. It had the same motor and bracket with a rubber band belt on it. So it's probably original.
So since I don't have any funds to spare, and I didn't have a bracket that would work, I drilled a second hole at the bottom under the slot, then used files to remove the metal between the holes and the slot making one really long slot.
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This gave me nearly 3/8" of adjustment, maybe a bit more.
As luck would have it I had a used belt that fit within the new adjustments, so I know I can get a new belt.
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Now I'm not stuck using the rubber band belts.
I feel better now. 
Joe