I have just replaced the 2006 R1 engine in my roadster with a Toyota 3sgte 2 ltr turbo mated to a 6 speed Altezza gearbox.
http://www.lotus7.co.nz/forum/download/file.php?id=1030
Everything fitted in nicely, and it is a pleasure to drive both on the road and the track. I have found it turns in better on the track with the heavier engine, but it doesn't sound anywhere near as good as the R1 did!.
I also went for a few laps as a passenger in a Rover V8 powered 7.... understeer city! As you turn in, the howling from the tyres increases, but it doesn't go round the corner....
Stick with a powerful 4 cylinder.. they work well in the roadster chassis..