Nikasil coating breakdown

It's common for the oil system in a gasoline/petro engine to overflow if it has been running with diesel fuel (Or Jet A/ kerosene) because it seeps past the piston rings and dilutes the oil system. Unfortunately, Rotax has no information on what is required when someone accidentally runs the engine with diesel fuel Or Jet A /B/ Kerosene.

The pitting you found on the cylinder wall is not from the fuel contamination, it's a breakdown in the Nikasil, and that cylinder and any other cylidner in that same condition need replacement.

Since the diesel diluted the engine oil, you may have done extensive damage to the crankshaft bearings and other interconnected parts. Diesel also lowers the octane in the fuel, and you may have signs of detonation on the pistons.

A full teardown inspection of the engine is highly recommended!

I very much doubt that there would be any significant diesel contamination of the crankcase/sump oil. 

Why?:

The carburettor float/valve system is there, in part,  to prevent raw/liquid fuel (irrespective of type) from enter the combustion chamber (assuming its working correctly)

It would be unlikely that the engine would run with a high diesel:petrol ratio - no run/combustion no chance of diesel entering the crankcase.

The most likely outcome of a  diesel shandy is either no start / start with difficult and much smoke / engine may continue to run after ignition shut down (dieseling).

If Geoff's oil tank overflowed it would likely be due to faulty float valves, faulty rings, overfilling of oil system, malicious intent. 

I‘d suggest to monitor the faulty spots over the next 25 to 50 hours of operation before tearing the engine apart. In case they get visibly larger you would need „a plan“ to go forward. Otherwise keep monitoring the spots when performing the annuals.