As an electrician with over thirty years experience working on old motorbikes and military vehicle electrics I would recommend if your build budget will allow to get Powerdynamo to build you a new alternator/mag rectifier set up.
The units they make are a piece of cake to fit and time up, the power output is great both for spark and charging, go for a 12V system and you will be trouble free for years to come.
I know the purists say it is not right to uprate systems to 12V but I have never found one of them at the side of the road at night willing to help when it all goes to pot.
I have no connection to Powerdynamo not even sure what country they bode from but there products are very well made and I put them on par with Motoplat systems without the huge price tag.
Best of luck