try getting ahold of southern cross sports, there in melbourne and they helped me out when i had mine (2 years ago) they should have the part.
they also had an ebay store...
dont forget the gasket also
that sounds ok to me for a cold day. but for a warm day more like 2-5mins

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You can test the thermo you have, take it out and inspect it first. if its open then its stuffed,,,,,,
If its closed drop it in some boiling water and it should spring open.
Or because there an old bike if youre going to the trouble of removing it you might as well change it
Do you have a manual for your bike?