So I get why the rule is written the way it is. In my case, I don't think any opponents would be unduly confused, and I paid for the model, but I can see how FF might find it problematic to write a clear, concise set of rules that covers 'you can kitbash stuff as long as you don't do it too much or confusingly or in a way that means you don't buy our products or that you detract from the 'cool value' of the game to observers' (tournaments are marketing venues, after all) without giving rise to endless annoying and legalistic arguments by That Guy at every tournament ever. Or someone might take a lump of clay and pinch it into a vague X-Wing shape and thus avoid buying more X-Wing expansions. For example, I could see it being a problem if someone kitbashed a couple of TIE Interceptors to have the same wings as standard TIE fighters, as it would be hugely confusing for their opponent. Since the no-modification rule is completely meaningless as far as game mechanics are concerned, I reckon it probably exists for some other reason. If this part of the figure would touch another figure or obstruct its movement, simply add or remove one peg from the base to prevent this situation and continue moving as normal. Some ship figures extend beyond the edge of their base. I've obviously got no special insight into Fantasy Flight's decision-making process, but here's the relevant bit from the current edition of FF's tournament rules:
