Hi GrantV,
Without looking it up, I think the difference in bore diameter is 4mm, and not 5mm. As I have previously said, the press blurb at the time of the launch mainly referred to the larger bore size, and they forgot to mention all the other changes. The 6.9 block's skirt has been extended to below the centre line of the crankshaft in the interests of greater strength, but for all I know that may only be , say 2mm, but it could be 15 - you know what I mean ?
Say, if you were to machine the bore by 4mms, the idea would then be to use 6.9 pistons. So, you could buy new - and you would find that they would have to come from Germany probably at about $US 1200.00 per piston, and even Companies like Autohaus Arizona if they don't have access to aftermarket parts, are quite close in their pricing structure to the OEM price. Forget Australia - our prices are sky high.
Next, you could then also consider used 6.9 pistons, but you'd still have to purchase new rings. There are some American rings that could be cross referenced, except for one ring where the 6.9 piston has a wider groove and, as far as I know, nothing else will fit, so you're back to original and the higher pricing structure. In addition, you may have to look at conrods - is there any diffrence between the 6.3 and 6.9 items ? At what point are the gudgeon pins situated ? Until you have comparable parts side by side, you may well be subject to a lot of guesswork.
Your last option is to fit a 6.9 engine to the 109 chassis car. It's been done with 107's, so I am sure that with money and the right people the job could be done. There is a post in "Topklasse" that refers to just such a conversion. Regards Styria