hi giorgio!
there must be a reason why the gasket is blown!
check the threads of the cylinder head screws!
was the oil level o.k.?
yes, it's possible to put metal bearings (sorry, i don't know the correct english word).
http://www.wunderlich.de here in germany offers this work, but it's expensive!
it might be cheaper to look for a second-hand head. right now somebody offers a cylinder and head in
www.ebay.de: search for xt 600.
current offer is 105 DEM.
yours
klaus