I need a new handlebar switches for the 83 harley fxr, and i saw you can but aftermarket ones one ebay, but are the turn signal momentary as in hold to keep turn signal on, or do they act like an on off switch?

 

You have to keep the button pressed for the signal to blink, not push-release-push-release etc
That's not because of the controls design, but just how the wiring works
You can buy aftermarket modules for automatic signals (press once and it continues to blink), self cancelling etc
I believe (please somebody correct me) that HD signal worked this way until late 80s-early 90s

 

that's why you never saw a Harley going down the road with the blinker going.

 

I have the self-cancelling module by Signal Dynamics and it works perfectly. The worst part about replacing the switches is running the wires into the headlight - assuming you have the stock harness.