Open a .webarchive Modify It and Save It

I'm developing an app for Lion and what I want to do is open a .webarchive file, modify a snippet of the DOM, and then write out the modified DOM to the same file.

Here is my code thus far. It opens the webarchive, modifies it, and then saves it back to the file.

    NSString *archivePath = @"/Users/tigger/Library/Mail/V2/MailData/Signatures/1216DD8D-C7E2-4DE1-9FCD-0A9A3412C788.webarchive";
    NSData *plistData = [NSData dataWithContentsOfFile:archivePath];
    NSString *error;
    NSPropertyListFormat format;
    NSMutableDictionary *plist;

    plist = (NSMutableDictionary *)[NSPropertyListSerialization propertyListFromData:plistData
                                             mutabilityOption:NSPropertyListMutableContainersAndLeaves
                                                       format:&format
                                             errorDescription:&error];
    if(!plist){
        printf("no plist");
        [error release];
    }else{
        NSString *s = [NSString stringWithUTF8String:[[[plist objectForKey:@"WebMainResource"] objectForKey:@"WebResourceData"] bytes]];
        NSString *new = [s stringByReplacingOccurrencesOfString:@"</body>" withString:@"hey there!</body>"];

        [[plist objectForKey:@"WebMainResource"] setObject:new forKey:@"WebResourceData"];
        printf("Archive: %s", [[plist description] UTF8String]);       
        NSData *data = [NSPropertyListSerialization dataFromPropertyList:plist format:NSPropertyListBinaryFormat_v1_0 errorDescription:nil];
        [data writeToURL:[NSURL fileURLWithPath:@"/Users/tigger/Library/Mail/V2/MailData/Signatures/test.webarchive"] atomically:YES];

    }

The problem is that the resulting webarchive is invalid. The original looks like this:

bplist00—_WebMainResource’  
_WebResourceTextEncodingName_WebResourceFrameName^WebResourceURL_WebResourceData_WebResourceMIMETypeUUTF-8PUdata:O<span class="Apple-styl