There are a lot more quite important differences between the Series 2 & 3 engines. Not only that, there are significant differences between the American Series 3 and the European engines.

Where you have currently got to, this is significant in that you've already purchased pistons.

The basic Series 2 & 3 engines have (well documented) differences in conrod length, piston compression height and pin diameter. What is less well recorded, but probably of less interest to you, are that these are also different in both the ST170 and RS (Turbo) Focus Series 3 based engines.

What is documented, but most people miss, is that the American engines also have another set of dimensions. In general, you can fit matching sets of rod/piston in either Series 2/3 blocks, but for example your pistons will not fit on European rods.

So you're going to be looking at another VAT bill, but even with that, with the current $/£ rate everything is still cheap.

Good Luck.