And Tom also did a supercharger version. That was a very nice car.
In thinking through engine spec for your purpose, a couple of questions back to you and the context for why you should think them through:
1) What class of racing are you wanting to be in? Is there an advantage between a 2L being in one class versus a 2.5L being in another class?
2) What style of driver are you? Do you like a frenetic drive, furiously rowing the gears, to keep the car high in the power band? If so thats a 2L style of engine. If you are more measured in driving style then having a truck load of torque to launch out of corners will point towards the 2.5L engine.
3) What rear end are you looking at - IRS hopefully? The 620R is limited by its deDion rear end. Personally it has so much power it cannot put it down. The IRS helps tremendously to put the power down and makes the seven feel like a bigger car/more planted.
You may also find this thread below helpful as I lay out how I was thinking through a similar project, albeit you are being much more sensible than I am!