I'm not sure how well hand laid FG would bond or if it would even provide any extra strength. I think your best approach is replacement as preventive maintenace every 60k miles.
Or buy the aluminum radiator and expansion tank from Zionsville Autosport for $1300.
2013 F30 328i - Melbourne Red
2011 E90 M3 - Monte Carlo Blue
2004 E46 M3 - Imola red