Battery has nothing to do with the engine running on a dt, It will literally need one wire from the mag to the coil to run. Bought a field bike for spares in just that configuration.
If you have a good plug, coil and lead then continuity down to the mag the only fault there will be the condenser or the points need adjusting or are fouled.
I myself used to run the condenser outside of the mag on dt's i have had in the past, may reduce efficiency slightly but save a whole lot of ball ache when they fail, and far easier to change out on a regular basis saving the points burning through a weak condenser. Condensers do fail, likely the weakest 'point' of the system.
May just be a bit of fluff in the jaws of the points.