try taking the cover off and shifting it into reverse watching the cable to see if it moves if it does and while its still in reverse go down and see if you can turn the prop by hand, if the prop still spins or if it doesnt move its likely out of adjustment or as you said you have a broken cable but first check the linkage inside the control box to make sure you dont have a broken cable end
check the simple stuff first ... ya know?
if every thing looks all there and intact take the cable loose from the motor shift it manually into neutral and the adjust the cable to where it will slide onto the little pivot WITHOUT moving the shift linkage may take a few tries but its pretty straight forward
can also try pushing the shift lever on the motor manually. had the same issue with reverse on mine, ended up having to replace the cable. basically wasn't getting enough movement, but could push a little more by hand to get in gear and prove lower was good
