How important is the rear brake

Been running without one for ages. Just stock front rim brake, swapped to operate with the right hand (my dominant hand) lever. Now that Ive motorized (48 or 49 or whatever CC) I want to upgrade the front to disc brakes for safety. Should I even bother getting a rear brake on there again - and if so, is it worth upgrading that to disc as well, or just run stock in the rear and disc in the front - first post BTW - and an instantly addictive hobby motorbicycling

Yeah, kinda what I figured I should do. Better safe than sorry. I've been keeping the speed way down in any kind of traffic situation and staying off the main roads so far. I think once I get my front brake upgraded to a disc I'll swap the stock setup from there to the back. That should give me some solid stopping power I'd think. And maybe I have more tuning to do but 30 MPH/48.xx KPH is faster than this thing will go on flat ground. Just under 40 KPH after holding WOT for a good while is where she maxes out.

Thanks for the advice folks.