Improving a permanent magnet alternator. Replacing magnets / reducing air gap
If the magnets are epoxied as securely as they should be to resist centrifugal force and vibration, removing them may be quite difficult. However, if destroying them in removal is acceptable, it can probably be accomplished. Since grinding may be necessary, precautions should be taken to prevent dust from being inhaled by anyone.

Attaching another layer of magnets with epoxy might be possible. The rotor would need to be re-balanced.

If the air gap is excessive, reducing the air gap may be more effective than adding magnet material.

I would think the speed at which the generator produces a useable voltage (starting speed?) would be reduced not increased.