Some of the previous answers are obviously from people who are guessing rather than having any precise knowledge.
Here are some FACTS....
1. Diesel fuel is a natural lubricant. It WILL NOT cause cylinder scoring under ANY normal operating circumstances.
2. Diesel has a CETANE rating, not an OCTANE rating. The CETANE rating is the effective opposite of OCTANE rating. CETANE rating is the time lapse from injection to fuel ignition in a compression ignition engine. A high CETANE rating means a longer delay from injection to ignition of the fuel.
OCTANE rating is a gasoline/petrol rating, of a spark ignition fuels, resistance to detonation. A high octane fuel has good detonation resistance, and a low octane fuel has poor detonation resistance.
Simply put... if you use low octane fuel in a high-performance engine, you will experience detonation more quickly than when using high octane fuel.
3. Diesel fuel contains much more energy than gasoline/petrol. This is the reason diesel engined vehicles get better economy than gasoline/petrol-fuelled vehicles.
It also contains a vastly increased amount of carbon and other pollutants. For this reason, high detergent level oils are used in the crankcase of diesel engines, to cope with the massive production of carbon and other pollutants that are a by-product of diesel combustion.
4. Diesel can be used in spark ignition engines, if so designed. Allis-Chalmers, the tractor manufacturer, produced a spark-ignition diesel tractor engine in 1936. It was a low-compression engine whereby the diesel fuel was injected, and then ignited by a spark plug. Special spark plugs were used to combat fouling, and a low pressure fuel injection pump and injectors provided the fuel.
The problems with this engine that made it less than successful, were high fuel consumption, spark plug fouling, and high crankcase oil dilution, as the diesel fuel did not burn 100% successfully with this setup.
The greatest single problem with using diesel fuel in an engine designed to use spark ignition fuel, is the substantially increased energy content of diesel fuel, will lead to higher combustion temperatures, and therefore engine overheating.
Engine overheating results in oil thinning and oil breakdown, so it cannot lubricate properly. Engine parts wear more rapidly in an overheating engine. Engine overheating will result in cylinder wall scuffing, as the oil film on the cylinder wall breaks down.
In a worse case scenario, engine seizure will result, when the bearings commence to gall, and the bearing metal sticks to the crankshaft journals. The pistons will melt and gall, and seize to the cylinder walls.
The second greatest single problem with using diesel fuel in an engine designed to use spark ignition fuel, is rapid carbon deposit buildup, due to the vastly increased carbon content of diesel.
When diesel burns in a spark ignition engine, rapid carbon buildup results, which can result in rapid spark plug fouling, ring sticking, cylinder bore glazing, and exhaust port fouling.
The indications are, from the reduced oil pressure, increased oil consumption, and the reduction in differential cylinder pressures, that the Rotax has been seriously damaged by overheating and carbon buildup. You have no choice but to totally dismantle the engine and rebuild it.