"Perhaps, depending on how many Rotax engines we are talking about, we could have a website with an aircraft reg and a Rotax serial."
What would that achieve, aside from informing all of the villains what serial numbered engines are where?
There must be a number of applications which "could" use a 100 hp petrol engine, most won't be aviation or even given a proper registration. Think hovercraft, dune buggies, jet ski, anything else people dream up to build in their garages.
Some aircraft types don't necessarily have to use a Rotax engine either. I know of more than one type that may use a Rotax or a Jabiru or a UL or a D Motor or any of several other types.