You can't build a new reshortened chassis unless you want to put the buggy through as a Full ICV and comply with all ADR's and testing.
Depending on the rust - you can replace sections or repair the existing chassis, but without any real rego history or engineering paperwork, you will probably be wasting your money as you may not be able to eventually register it.
There is a guy down there at lakes entrance that recently bought the Astrum Moulds -
http://www.manxclub.org/phpbb3/viewtopic.php?f=15&t=1399&p=12473#p12473
There is a website and email listed under his user profile.