yeah, you can buy replacement keys for that at any local mower shop or TSC.
and toystory, yes you are right, you have not seen many garden tractors with 6V systems because garden tractors and riding mowers are different. Garden tractors are used as a working "tractor". They need the extra battery power to use for an extra PTO system. A lot of people think Garden tractors and riding mowers are all the same. Well, in most sense, yes, they are. But, garden tractors are....well.... i don't really wanna say "more of a heavy duty" tractor, but they are. They are used more for work. If you are like me, well, you don't really care, you'll use your mower for anything as long as it gets the job done.