You won’t find an RPEQ for something designed overseas - that’s a local thing. What’s more engineers will only certify their own designs - so you can’t get someone here to certify it complies with our regulations ( it won’t meet the Building Code of Australia, for example).
An alternative view is the owner erects something at own risk, on the assumption that when they leave the property it is disposed of as being a non compliant structure it can’t be on the title. Council could force its removal at any time.
I have the assembly instruction booklet, but they have no plans of the observatory, just text with some photographs to aid in assembly. If you don't have the booklet and if that's any help I can photo and email the pages.