G'day Marcus,
Welcome along. I had a similar issue long time ago, would only ride about a mile and stop.
As there was spark (even hot) I blamed and rebuilt carby, but no go.
Finally conceded to rebuild magneto, which had, I was told, the original wiring and condensors -all shot, now it's fine.
Also check the plug leads and caps, had a minor issue with mine the other week, and replaced leads- now good again.
Cheers duTch