A planeswalker that becomes a creature due to Spark Rupture is no longer a planeswalker. It can no longer be attacked, and damage dealt to it won't cause it to lose loyalty counters.

A planeswalker affected by Spark Rupture loses all abilities it has, not only loyalty abilities. The resulting creature may gain abilities after Spark Rupture enters the battlefield.

While Spark Rupture is on the battlefield, planeswalkers will enter the battlefield as creatures, but with the appropriate number of loyalty counters. For example, a planeswalker that normally enters with three loyalty counters on it will enter as a 3/3 creature and have three loyalty counters on it.

The resulting creature's power and toughness will change if the number of loyalty counters on it changes.

If a planeswalker-turned-creature has all loyalty counters removed from it, Spark Rupture's effect will stop applying to it. It will momentarily return to being a planeswalker, but as it has no loyalty counters, it will soon find itself in the graveyard. If this happens, you won't have priority to activate any of its loyalty abilities before the planeswalker is put into its owner's graveyard.

In some unusual cases, Spark Rupture may enter the battlefield during a combat in which a planeswalker has been attacked. In such a case, that planeswalker will be removed from combat. If blockers haven't been declared yet, the resulting creature may block. Creatures that were attacking that planeswalker remain attacking and can be blocked, but they won't deal combat damage if unblocked.