Oil Pressure Relief Valve

I would begin by beating him with a copy of the Rotax heavy maintenance manual. It won't fix your problem but it will make you feel better ;-)

Ideally you need to find experience of someone who has done the same and they know the outcome. I would phone around some of the Rotax repair shops, this must have happened before or a situation where the relief valve got stuck and also resulted in no pressure.

My experience of oil flow problems is that the bearings often suffer first, so if the engine isn't siezed then that's a good start, I would then pull the magnetic plug and check for any ferrous material and then cut open the filter to see if it has caught any bearing material.

Beyond that, I don't know what to suggest, it's a very unfortunate situation.

Time for the experts.  Call Rotech, Lockwood, LEAF, possibly others.

Concerning Kevin's comment:  my fear is that, without measurable pressure, there is little flow with which to transport any "new" metal to the filter, mag plug, or oil tank.  They might look perfectly clean, in spite of considerable damage.  An oil system flush/purge might move metal out to where you can see it, but even turning that engine by hand may not be advisable.  Certainly an experienced borescope study of the cylinders is in order.  Even if all that produced "good news", how would you feel about flying it? 

Chris makes a good point, there may be no evidence of any damage in the filter or on the magnetic plug. How about removing the lower crankcase oil return banjo and having a look at what's inside? At least if you find evidence of damage you can just pull the engine and send it off for repair. The bigger problem is what to do if you don't find anything. Is it possible to check the engine sufficiently in-situ without removing and stripping it?

Well this could be bad.  Start by finding out if you seized it.   If you are feeling it is sticking at all then remove engine and sent it to an overhaul shop to stirp it and replace the check/crankshaft or crankcase, depending on how long it ran it may have turned a bearing.  Looking for oil to show material is unlikely to work in that it was a short run and hopefully not under load.  The most common failure in an oil starved Rotax is failure of the lower conrod bearing (commonly will be number one rod also) as they are the ones under the most load.  The lower rod bearing is a brass alloy.  

Don't cheap out and go to a backyard guy if you decide to have it stripped, the lower end is something you only want a very experienced person working on. 

Cheers

This is why taking a Rotax class is so important for owners and mechanic's. Even if you don't do your own work take at least a service class so you'll know when someone working on your engine is doing it right or wrong.