Firstly

to XT660.com!
The mudguard/fender is from a Brazilian market XTZ250X like this...
It has had the plastic stanchion guard carefully trimmed off.
You'll never find one of these in Europe and most people don't even realise there is such a thing as an XTZ250X in existence! You could possibly order the part from a Yamaha dealer in Brazil?
The forks are I think from a WR or possibly a YZ?