i wouldn't use a cracked wheel if repaired - maybe only as a spare. you don't want the wheel failing on you again, perhaps more catastrophically.
where is the crack? you certain it was not due to an impact? if it's somewhere along the rim, especially along the inside of the wheel (where it is weakest), it is unfortunately most likely from some form of impact, and unlikely to be covered by warranty unless you can prove otherwise or the dealer displays goodwill.
if it's cracked somewhere like near the valve or where it would not normally crack as a result of an impact, it could be covered by warranty.
i hear the original 18" wheels are expensive. as a comparison to the OZ, quite possibly UK$435 each.
be very careful about aftermarket wheels and their fitment. OZ typically do not have 5x98 PCD wheels (only 5x100 being the closest) so you'd need to use wobbly bolts. offset needs to be close/exact (eg: +35mm on 7.5" wide wheel), and most importantly, they need to clear the front Brembo calipers behind the spokes. hence, fitment is very unusual and extremely few wheels fit. many would fit the JTS/JTD, but not the V6 because of the front brakes.