Yes, if the earth brush can be persuaded up the HT
pick-up, but there are potential problems.
If you also have an auxiliary earth brush on the inner
face of the cb backplate, everything should be fine. However, if not, removing
the sole earth brush means that the HT current will need to find another way
from the engine back to the magneto armature. If the drive pinion/sprocket is
metal, then the HT current can flow through the drive gears or chain, which
should be OK as a temporary measure. However, if you have a tufnol or plastic
drive pinion on the magneto, the HT current will probably end up passing through
your bearing races and balls and jumping the outer race insulators. The
insulators can be damaged very quickly, and the bearings can also be damaged in
the long term.
If you do have an auxiliary earth brush, you might try
using that instead as the makeshift HT brush. But you may find that it does not