Alan

The diodes are replaceable - but they need to be disconnected to fully test them - there are usually at least 12 diodes (some times 18 or more). Depending how many, how the diodes failed and the charge state of the battery it's also possible to have damaged some of the stator windings during a failure . A good old school Alternator/Motor shop will be able to evaluate the viability of repair. Unfortunately there are very few of these shops left anymore. Generally anything is repairable BUT the parts/labor costs to do so may be prohibitive... worth checking out if its generally in good condition (was previously working fine and bearings are OK).

Alan