Originally Posted by Kev

It would be cheaper to strip & rebuild, then to buy a second hand motor from over seas & pay import duty + shipping cost.

I would charge Oz $500 for a loose motor strip down & rebuild + the parts below, so find a local mechanic to do your motor at home.

So if you pull your motor out a Bike shop would charge around $800 to $1000 labour. + the below parts.

It took 5 min to get the below prices, if I look harder I am sure I could find the rest.

Head gasket

Base gasket
http://www.ebay.co.uk/itm/GENUINE-YAMAHA-CYLINDER-BASE-GASKET-XT660X-XT660R-/280772448415?pt=UK_Motorcycle_Parts&hash=item415f57909f

Clutch gasket
http://www.ebay.co.uk/itm/GENUINE-YAMAHA-GASKET-CLUTCH-COVER-XT660R-XT660X-/380341310263?pt=UK_Motorcycle_Parts&hash=item588e1bd337

Stator cover gasket
http://www.ebay.co.uk/itm/GENUINE-YAMAHA-GASKET-CRANKCASE-COVER-XT660R-XT660X-/280739613491?pt=UK_Motorcycle_Parts&hash=item415d628b33

A few gears

Sealant
http://www.ebay.co.uk/itm/YAMAHA-XT600-XTZ600-XT660-XTZ-XT-SILVER-GASKET-CEMENT-/270901116296?pt=UK_Motorcycle_Parts&hash=item3f12f6fd88

2X Exhaust gaskets
http://www.ebay.co.uk/itm/YAMAHA-XT660-XT-660-ALLOY-EXHAUST-GASKET-/400246641368?pt=UK_Motorcycle_Parts&hash=item5d308f12d8

Or a complete gasket set
http://www.ebay.co.uk/itm/Yamaha-XT-660-R-X-Z-Tenere-ATHENA-Motordichtsatz-Dichtsatz-Gasket-Set-/290664492935?pt=Motorrad_Kraftradteile&hash=item43acf42f87