I'd say your definitely correct , your better changing like for like, I'm just saying it can be done. I've also seen a heating merchants advertising pumps to replace riello, not sure of the price or that they are any good.
I do know exactly what ur saying about the sulphur but there did seem to be more problems with the riello pumps than the danfoss pumps, also I would personally come across a lot more riello burners than any other makes.
Rdb and mectron oil pumps are slightly different but I have come across them being used on each others burner by changing the connector that feeds the nozzle from the pump. The stems are different but they will work. What I didn't know was that you can adapt a danfoss 21r3 to work on a rdb...