I think I would be looking under the flywheel blue. Check the points and condensers and make sure the coils are good. If they havn't been replaced for a long time, chances are they are cracked and aren't working anymore.
I think the HD-25 has the magneto system from before the late 50s/60s/70s universal system. The coils in it are apparently far more reliable than those. Probably just needs the points filed and gapped.
