As I understand it Bongo, the problem is due to the fact that the nacelles, engine mounts and plumbing are/were designed specifically for the early Herc which precludes them being re-engined with something from P&W for example without major rework. It also means that late model Hercs (from, for example, a Hastings) cannot be used either. That said, there are photos around showing Beaufighters with merlins! Mark II's I think