Not necessarily unrideable if only one gear tooth were missing. When I acquired my Brum and gave it a test ride, all appeared normal in the gearbox department. It shifted and ran in all three-gears without any indication of trouble or noise. Yet on disassembly it was missing a tooth. Can not remember which position it was in after all these years, but just dug out the broken gear from the box of left over bits and note it is (or was ) 22t and no dogs, so one of the lay shaft gears I presume. The missing tooth was not in the gear case on disassembly!
Also know of friend who ran a Model A Ford 3-speed clash box some distance without a tooth or two (though larger diameter gears.) He also drove from Long Island, NY to Philadelphia without the bottom in a trans either, just a rectangular gallon tin oil can cut to serve as an impromptu sump and wire up in place! The same fellow, as well as my father, broke teeth off of the ring gear pinion on the rear axles of their Models As (cheap transport in the fifties.) That WAS definitely noticeable, but by careful modulation of the throttle they were able to nurse the cars home from work so they could be repaired. Father’s had two teeth missing by the time he got home! Amazing what you can get away with.
But I digress. The missing tooth did not cause the kick start to jam in my particular case the two dozen times I had occasion to use it before ripping it assunder for restoration. Though perhaps the very next kick it would have.