How much do you know about this?
So let's see... you're already thinking about lowering CR.... why? How much boost are you wanting to feed it?
How are you going to deliver the extra fuel?
Are you going to run some piggy back or full stand alone system? Or custom tune?
You going single or twins or what?

 

The 4.2 is an un-sleeved Alusil block. They are very tricky to hone without destroying the thin silicon crystal lining on the bores. If boring is required, you pretty much have to sleeve the block, but the lack of space between the bores makes this difficult as well. Turbo or supercharging these blocks has been done, but the cost doesn't justify the outcome.

You can read up on the block and reconditioning here:
Audi 's Alusil 4.2 Engine Rebuild Resources part 1

AMTuned and JHM Motorsports are who I would reach out to for more information on building these blocks, but it appears that they have shifted their focus to the FSI 4.2 in the S/RS models

 

Based on what he said, he "intends" on using the pistons and rods straight out of the other engine since they share the same bore but have a different stroke ..... but there's a lot more to this parts swap, so I'll wait to hear the plan.....