jetjr
In a general sense I agree with you, both on owner responsibility, and that RA shouldn't have to pay for every fix to a mistake...
but in this specific case, I disagree totally. Having advice by Lightwing, the Rotax distributor and RA, it would be a very suspicious person to seek another opinion - and where would you have found it? Most most GA engineers would have said "Well, normally, umm...but this is a strange exception where 25 reg ultralights don't have a type certificate, so I do not know". So where else would you go? CASA would probably refer you to RA. You could pay a lawyer to research it for you - at rather more than the cost of a new engine!
I also don't think RA should normally pay for such mistakes, but in this case, they seem to be brushing it under the carpet and making it an individual aircraft problem, rather than seeking a general solution to what was a systemic problem. And they did get 10 years of rego money for every single aircraft, where just one crash could have exposed them/us to a very expensive court case.
Lightwing haven't come out of this looking that clean either, as far as I can see.
As Dafydd says, it is very much caveat emptor, but you would have hoped RA would be less secretive about the issue, and more involved in a solution rather than just telling owners their aircraft is unregistrable.
Dodo