I'm sure in katakana it'd be something along the lines of "he-a-do su-wa-po" or something equally stupid looking :P

Though I think translating it directly into Japanese wouldn't work-- 'head' would be 'atama', but I doubt 'thwap' has a direct translation, unless we go for sound effects in Japanese. Then lord knows what sort of bastardizing we'd be doing to the language.

*is finished being language-retarded*