Non Rotax Solenoid, use at your own risk!  Rotax 2-cycle 503 engine wiring Diagram for my Starter Relay / Solenoid.  I have a push button starter switch which supplies the positive side of the relay coil.  A separate ground wire is attached to the mounting tab of the solenoid which screws into the engine block.  The other end of the ground wire is attached to another ground point on the block.  This method insures the relay will be well grounded.  This starter relay/solenoid was purchased at our local automotive parts house for approx $12.  (CarQuest #84010)  The OEM Rotax solenoid price quoted was nearly $200 delivered.

Note:  I was instructed to attach the control wire from the starter push button to the (I) connection on the relay.  This was incorrect.  With a positive feed from the push button that wire is connected to the (S) terminal.