I obtained a Level 50 Heatran, and based on leveling it to 100, checking the Move Relearner, and past games, its full level up movepool is likely as follows:
Start - Magma Storm
Start - Heat Wave
Start - Earth Power
Start - Iron Head
Start - Fire Spin
Start - Ancient Power
L9 - Leer
L17 - Fire Fang
L25 - Metal Sound
L33 - Crunch
L41 - Scary Face
L49 - Lava Plume
L57 - Fire Spin
L65 - Iron Head
L73 - Earth Power
L81 - Heat Wave
L88 - Stone Edge
L96 - Magma Storm