I too investigated the Rotax problems on this aircraft. PFA engineering are very happy with the reliability of the Rotax in the lighter weight aircraft, “ brilliant reliability record” was the statement. I had come to the conclusion the problem was in the installation on this one aircraft type.
The cost justification is very easy. I can get a 912 for under 8000 inc vat. At the moment I am running an O360. The 912 will cost me £12 per hour and the O360 is costing me £39 per hour in fuel. Over a 10 year period that amounts to £27000 in saving assuming the differential remains the same. If the cost of fuel rises by the same amount over the next 10 years as it has over the last 5 then the cost saving would several times this. I do about 100 hours a year.