parts:
try at
www.ebay.com (search: XT 600 or XT600)
or use the 'kleinanzeigen/suche' forum at
www.xt600.de/boards. you can assume that most of the folks there are able to understand english.
gearbox/sprocket:
YES!!! better use 14/45 or even 14/48.
TÉNÉRÉS are not build to spent their lives on motorways... ;-)
regards
klaus