First off, blocks are the same height, and the maths says that if you did it (998 standard bore block, 997 crank) you'd have a 1065cc engine.
If you did go through and do it, the interesting bit would be which pistons you used. The poor old 997 had everything against it - the stroke of a 1275 and the bore of an 850. So it had small valves because of the 850's bore, and a long stroke like a 1275, exacerbated by the small bore (so the bore/stroke ratio was even more undersquare).
Even just rebuilding a 997 stock is a challenge now, because there weren't many of them made worldwide (and the engine size was unique to the Mini), so there aren't many overbore pistons available. They shared the bore size with the 850 (62.48?? bore), but a different deck height (height from gudgeon to top of piston)... So you can't use 850 pistons for a 997 rebuild.
Given that BMC had a habit of changing deck heights, you'd need to trawl through all the manuals and see if the 998 or 1098 deck heights were useful, or within ballpark. If they were close but a higher deck height, you could machine them to work. The odds are, though, the cost of finding suitable pistons, or, more likely, machining pistons to be suitable, would be more than the cost of buying a 1098 crank/rods/pistons. I could easily be wrong, but my understanding of what Dr Mini said would mean that a 997 piston has a higher deck height than a 1098. I'm assuming 1098 conrods to go with the 1098 crank, and 997 spec pistons to go with the bore - although 850 pistons could have been used, which would destroy my assumption.
Anyway, if 997 pistons had a higher deck height than a 1098, that doesn't bode well for finding a suitable piston.
Could be very low compression!
I'd imagine you're asking because you've got a 997 crank..? What else have you got..? I'm not after anything 997, but someone else on here might be interested. They're pretty scarce nowadays, but they're only really of value if you're doing up an original 997 Cooper. Even then, it's only financially worthwhile if you've got matching numbers. But you never know - I had an old 997 engine a few years ago. I bought it with the intention of putting into my car, which started life as a 63 Cooper (but wasn't very Cooper by the time I got it
). It wasn't the correct numbers for my car, or the guy I sold it to, but then a 997 Cooper is a rare thing, and there are many S's out there which, although they have an S engine, don't have matching numbers. Similarly, that guy (and previously myself) just wanted the right size engine in the car. There may well be someone who'd pay for what you've got - or trade for something else more suitable for your plans.
