The 2.0 Duratec might be a good option too. It can be found in plenty of Fords from the noughties and is all alloy, so lighter than the ST170 engine. Only 145PS as standard but there seems to be countless tuning options available. Caterham use a lot of Duratecs in the current Seven I believe so, if it's good enough for them, it might do what you need!