In those HSI units there is an optical sensor that controls the syncing. When it fails the HSI will no longer sync and makes it look like the gyro has failed. When it goes completely the rotating card just spins round and round. It is a common fault with them and would be my first bet. After that it is a gyro unit which from memory was about €300.
Beyond that I can't be much help, it's all a block box to me!!