Is my engine scrap?!

I do not have such experience like RL, RW or RO, but I think that you may have a problem with your gearbox as it is a most probable and usual source of metal particles found on the magnetic plug.

Interesting is that the black "sludge" like particles found in the oil tank did not clog the oil filter at all. Maybe you or the previous owner did use a not Rotax approved oil or strange (not approved either) oil additives? If so, it might result in the sludge build up in the crankcase (and in the gearbox), that is now being flushed to the tank....

Are your oil lines according to Rotax specification? Especially the one returning oil from the crankcase to the oil tank, as it has to deal with the hottest oil...

That were my thoughts. Hope they may help you.

Stefan 

I agree with Stefan.  Remove, disassemble, and inspect the gearbox.  Since you are not seeing metal in the filter, then if the problem is located in the gearbox, you may be able to flush the engine oil system as per the MMH and avoid a total rebuild.