2.0 litre to ST170 conversion isn't much quicker. With a few tweaks to the 2.0 litre they will be making more torque than the ST170's do anyways.

If you're going to do an engine swap do a worthwhile one and go turbo! RS engine swap isn't the most straightforward but enough people have done them regardless so there's plenty of info available. Or just turbo/supercharge the 2.0 litre as it'll take it easy peasy.