Hi there! Just to let you know the HTC Magic originally come with Android 1.5 but was update to 1.6 officially.
To date, I haven't heard of ANY HTC Magics coming installed with the Sense UI so I believe when you bought your phone someone has obviously rooted the handset and changed ROM's!
Check out
All Things Root - Android Forums and discover how to change ROM's etc. There are many ROM's available including cyanogen etc.
GOOD LUCK!