Starters... for starters.... there are two separate actions. The first action is to move the starter gear out (aft, relative to the vehicle) to allow the starter gear teeth to mesh into the open space of the flywheel teeth. On a foot starter, this action is entirely mechanical and requires no electrical power. The second action is the starter turning, which is initiated by the depressing of the starter button. This permits electrical current flow and allows the starter gear to spin, in turn spinning the flywheel, and eventually starting the motor.
When you disconnect the negative cable from the battery, the starter will not turn. After you disconnect the negative cable from the battery, open the hood and look at the starter. There should be a lever integral to the starter, along with a spring and linkage to return the foot switch assembly back to the 'starter disengaged' position. Push on the lever and verify that it engages the starter switch to fully depressed position. If the starter switch is fully depressed, then you may have a broken bendix inside the starter, which requires replacement. To verify, remove the starter and actuate the lever. When actuating the starter lever, the starter gear should physically move from the retracted position to the extended position. In the extended position, the starter gear should nearly touch the nose of the starter body.
It is possible that the 12v conversion was performed incorrectly, in that a 12v starter was installed and the larger 6v flywheel was not replaced. This may cause similar behavior. However, let's troubleshoot one step at a time.
