Regenerate attachment metadata

Is there a way to regenerate image attachment metadata? I imported posts from PixelPost, but the metadata is not quite right. It’s sufficient for some things (e.g. WordPress knows the image is there and has the correct URL), but I’d like to call the thumbnails according to the metadata.

The metadata resulting from the import looks like:
a:5:{s:5:"width";i:800;s:6:"height";i:529;s:14:"hwstring_small";s:23:"height='84' width='128'";s:4:"file";s:77:"/f1/content/kristarella/public/wp-content/uploads/20081020143128_wedding5.jpg";s:9:"thumbnail";s:35:"20081020143128_wedding5-200x132.jpg";}

but metadata from uploading through WordPress looks like:

a:6:{s:5:"width";i:400;s:6:"height";i:300;s:14:"hwstring_small";s:23:"height='96' width='128'";s:4:"file";s:73:"/nfsn/content/kristarella/htdocs/wp-content/uploads/protective_wrap03.jpg";s:5:"sizes";a:2:{s:9:"thumbnail";a:3:{s:4:"file";s:29:"protective_wrap03-200x150.jpg";s:5:"width";i:200;s:6:"height";i:150;}s:6:"medium";a:3:{s:4:"file";s:29:"protective_wrap03-300x225.jpg";s:5:"width";i:300;s:6:"height";i:225;}}s:10:"image_meta";a:10:{s:8:"aperture";i:0;s:6:"credit";s:0:"";s:6:"camera";s:0:"";s:7:"caption";s:0:"";s:17:"created_timestamp";i:0;s:9:"copyright";s:0:"";s:12:"focal_length";i:0;s:3:"iso";i:0;s:13:"shutter_speed";i:0;s:5:"title";s:0:"";}}

Thanks.