Re: what did you do to your motorized bicycle today?
That could very well be the problem... either they never put a key in it on the assembly line and somehow it held this long or the last guy lost it, these things will go for a while without a key if the gears and sprockets are a nice tight fit... but not very long.
I know on the centrifugal clutch kits the key can't be used at all on the crank because with the bevel gear removed and the shaft extension installed, all that keeps the shaft extension from slipping on the crank is just that slight taper on both parts, then when torqued down it don't slip. I've had my bike up to 38mph without a slip at that connection, but what did make it slip was when the gear separated from the clutch bell.
I would buy a replacement key, but buy a few more just to keep on hand as spares in case one goes flying out during disassembly or reassembly, they hold just fine once installed, but get lost real easy.