Magnuson-Moss - something similar
 

Is there some sort of federal law that is the equivalent to the Magnuson-Moss statute for Australia?

We have a Department of Fair Trading that the we can apply to if we have issues with companies providing goods and services.
They can act for consumers and have quite a lot of power when it comes to justified cases.

I think it becomes more difficult when traders are internet based.

I was thinking more on the lines of me, for example, buying a CBE but an engine component breaks such as one of the pistons explode - a bit OTT but it was the first thing that I could think of from the top of my head.

And what would you expect from the exhaust supplier? Are you buying a CBE that no other person has ever tested? It would be difficult to prove.

Its the same with Toyota, they would not deny a warranty claim on engine unless they could prove the exhaust was at fault.

It comes down to how much risk you will accept. You modify a car at your peril. Do the research and read reviews.