what gearbox you running ? if mtx75 can run a rs focus light flywheel same as std zetec 240mm check out puma speed or build cant remember which one know not cheap, so might want go with the ttv option from burton power as bit cheaper.
ps dont think can lighten the std st170 one as it is a dmf unit, oh another place to check is the states ebay etc as the the svt focus is the same as uk spec st170's so might do a light conversion