Did you try these folks?
http://www.moto-masterusa.com/
or these folks.
http://www.motostrano.com/supermoto.html
Then there is QTMI local to So-Cal. They may be
able to help.
http://www.qtmi.com/
And of course you asked the guys at Go-Fasters?!?!
http://www.gofasters.com/
Someone must have a GasGas SM front rotor sitting
around somewhere.
If all else fails. Let your fingers do the walking and start
calling the USA dealers.
http://www.gasgas.com/dealers.htm
A new rotor has got to be easier than trying to fit a
whole new hub. I am sure you could get a Tallon or
some other after market hub to work...if you found the
right bearings and spacers. Anything CAN be done with
enough research, $$.00 and time.
I am lazy and choose the path of least resistance.
Come back if you run into a brick wall and we can put
our heads together and see if we can figure it out.
Good hunting. Jeff