@Peter: Seen Hauke's S1 RST on a meeting? Probably Marktheidenfeld 2004?
Or read anything he wrote on www.ford-escort.de @ the forum?

This car?

1.6 RST bottom end
1.8 Sierra crankshaft
Custom made Wiseco pistons
Sierra 4x4 WM-Management

You'll just have to try which conrods you'll need with the higher stroke and then what exact measures of your pistons you'll need. Don't bore it up too much. 1.8 +x and enough of boost should be more power than your gearbox can cope with