If it were a magical item, it would never be destroyed after the casting, but it's innate to the model so the bound spell can't be lost. Same with the Anvil of Doom and Empire Battle Priests and the Empire's magemobiles. Some army book bound spells do cause problems on a miscast like the Wood Elf Sisters of the Thorn, but we are in the clear with Solar Engine.
This causes confusion. Don't let your opponents try to say you can't use the Solar Engine again.