Nobody makes custom cut Magnesium wheels that I know of... I can't even imagine what that would cost!
If I remember, a set of custom cut aluminum rims (chromed) ran close to 10g's from a manufacturer in Eastern Canada (Toronto?)... I wish I could remember who... He did one of Billy Lanes bikes. Rims were SICK!
All the custom cut rims are for show... They don't undergo any real structural testing like forged race wheels. Do the right thing and get some proper rims.