WHY????? The Cooper 998 only used a standard engine block so why make work
for yourself? The only real difference between a 998 Cooper engine and the
std 998 engine is the pistons, cam and head.....fitting the original style
of 1/2 Dome D top slipper piston into you newer block is only really
rebuilding the 998 but with out the rear tappet/pushrod covers, and
introducing the A+ block which some say is better, some say not....I'd
either freshen up the original 998 unit, as is. Or sell it to a
collector/resorer and use the funds to purchase modern "go faster" bits
which would be more suited to your engine model. Utillising the Cooper
piston isnt going to suddenly give you massize horse power or anything like
that, unless yo also spend the money and time to line bore the block,
balance it from top to bottom etc etc, ligthen the reciprocrating parts (ie
the bits that do all the moving) so it will rev better......and if you going
to to that much trouble and expence yo may as well use good quality new
stuff and have another engine built.

There is this belief that because it says "cooper" on the packet it will
sudenly become some fire breather monster....bad news it dont. Cooper is/was
a tuning house who built cars which where quick in their day, and still by
many standards are still, but unless all the fundementals of good engine
design and thermodynamics are followed all your doing is sticking bits and
bobs together and more than likely creating problems. You need to ensure
such things as the deck hight is spot on, compression ratios etc etc etc and
can yo get a suitable fuel etc etc, so save yourself grief and if you want
to go faster or what ever get a reputable engine builder to put together
somthing purpose designed and built for you. There are loads of such
companies in the UK, and Im sure Steve from DSN, whos a regular here, would
be able to point you in the right direction, which may at first seem like
more money, but I would almost gurantee you that at the end of the day will
be the much cheaper and more reliable way to go

just my opion anyway, but based on over 28 years of Mini ownership,
including a  66 Cooper 998, 64 Cooper S 1071, 67 Cooper S 1275, a 68 Mini
Jem GT racer, a 2000 Model S Works Rover Cooper and my own home built 1330
street "racer" a MK1 850 Superdeluxe 1967 & a MK1 850 1966 ....and a stack
of bills for repairs and work done to correct the cock ups Ive made in
trying to bodge together "go faster" mini's.
Oh and by the was I still own all the above listed cars...and NO none of
them are for sale!