Jack,
I have a 106 that my wife runs in the Giro. There are some major differences in the crankcase and altenator ignition. And also some minor differences in the Sears/Gilera models.
Gilera actually never made a 106, they made the 98, the 106 was a Gilera made "Sears" bike and had some minor differences to the Gilera badged 98's.
The 124's had a different cam drive setup from the 106 I think. And the left side engine covers are different. etc. Although I think you can bolt up a 124 top end to a 106...but I'm not sure.
JohnnyB