I know people have done them, but you have to ask whether its worth the risk, yesteerday we had a flywheel in which had a cracked and split clutch. the driver was unaware of any issue other than perhaps a clutch that was starting to slip.
basically the fly went blue and started to hairline fracture around the perimiter of the teeth, should it have failed id hate to have been around.
Lightened ones are fabricated from stronger, but lighter materials like chromoly in order to retain the strength and resiliance of a stock wheel but with less mass. and for all the money thats being saved id plump for a proper one time and time again