2010 Birel Monza Cassette and Bearing Question

I am looking for a 50 mm rear cassette and bearing hub to fit a rear axle on a 2010 Birel 30mm Monza frame.

Are rear cassette and bearings standard across European manufactures? IE could I use a OTK, CRG, Arrow cassette on a Birel kart. Please advise.

Can anyone advise on a hole patterns. I noticed some cassette’s are round and other’s are partially round and sit square in the cassette hub.

Thanks Guys