A light wt. flywheel is FUN, it revs up / down so much faster it feels like a race car.... I cant really explain it any other way, when most people shift they just time it, i have to blip, i cant help myself.
now as for machining it yourself.... i dunno, I don't think the would be heat treated or anything, but you better make sure you have no runout and that you can really balance it... if you don't know what I am saying, I would not do it.
my lame 2c.