That's the simplest type of rec/reg. If you need a replacement then any common rectifier for a 2 wire AC input that can handle 120Watts or more should do. Your type of alternator puts out around 90Watts. Not sure about the colours you mention but 6 volt is not relevant here. Most of the original supplied rec/regs have markings on the connectors like- G, G, B, M, R. G=giallio/yellow, B=Bianco/white, R=Rosso/red and M=Marronne/brown.
Yellows are the AC from the alternator.
White is an earth from the stator plate (a sort of belt and braces extra)
Brown is a switched live 12 volts.
Red is permanent live 12 volts Best via a fuse.
Some alternators don't have the red lead.
Please excuse me for any mis spelling of the Italian or the English.
There are some good explanations on the Dutch club website and there is a link to it from this website.
It's worth testing the output from your alternator before you condemn the rec/reg too.
Mark from Berkshire will be along soon to correct any errors in this.
