Hi-
Yes, you want to base model Mini pistons like this one:
2005-2008 MINI COOPER BASE CONVERTIBLE R52. 2002-2006 MINI COOPER BASE HATCHBACK R50. Engine Piston With Connecting Rod. Shown in pictures. No warranty on electric parts. All parts are tested before removal from the vehicle!
www.ebay.com
You'll have to bore the block to 77mm and cut the crank journals for the Mini sized bearings. It's not a big change to the crank but make sure your machine shop makes a generous radius at the ends of the journals for maximum strength. The Mini rods are a tiny bit narrower than the Opel ones so they float a bit on the crank journal. I was concerned about this when I built the engine but now that I've run it for months including a full 24 Hours Of Lemons race with this setup I can say it doesn't matter. The pistons sit a tiny bit above the block deck and with a stock 1mm head gasket you end up with a zero squish engine. Perfect fit. Do not deck the block.
As for the head, take a look at Honda K series valves. They are even bigger than the ones I used and can be modified to fit by a good machine shop. Depending on how much you cut down the head for compression you might have to notch the pistons a bit for valve clearance. With the Honda B valves I did not have to notch with a stock cam even after cutting 1.5mm off the head but the K valves are bigger.
Keep us posted on how you do. Just doing the Mini pistons alone will get you solid gains because of their lighter weight and lower friction design. If you can stand to look at a programmable ignition I would use one because once I figured ot out on my car it really helped.