Sounds like the commutators have been coming to bits. They don't like 'high' RPMs (hence why they are fitted with a bigger pulley than alternators) but even so, with a more or less stock engine, you'd be hard pressed getting enough RPMs from it to do this.
Another cause for them coming apart is high current flow through the commutator for a continuous period.
I doubt this would be the issue, but just to mention it. The Bearing in the front end is electrically insulated from the frame and this insulation must be maintained. Usually though, if it does short across this, the Bearing fails rather then the issues you are seeing, but, if it doesn't fail, then it will cause the armature to get VERY hot and fail in short time.