Try Sahib's Auto Electrics on the Hanger Lane Gyratory system (assuming they're still there - it's been a while).
They're happy to just replace the bits that need replacing and assuming the bearings and windings in your unit are all OK so it's fixable, it shouldn't cost too much.
If it's gone suddenly dead as a doornail, chances are it's the electronic regulator pack. If you're feeling keen they'll sell you one for a few quid and you can save your pennies. If you go this route, swap the brushes while you're at it.
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