Some of the conversions have transmissions, and obviously that would be preferable if it fits your budget and bike. But the torque curves on these is very flat once you are rolling, so you can "get away" without one, by making the same compromises as the no-transmission electric vehicles do.
I would love to see some transmission options, I know the old BSA and any divorced transmissions route,
retro-direct doesn't work on a diesel, some folks have converted 2 and 4 stroke bike transmissions (would love to see a build thread on that) by sawing off the cylinders,etc, others have made custom gearboxes...