You only need something that will fit in the keyhole. Sunday evening we enjoyed a lovely sunset sail but when the time came to start the engine and motor back, our only key was no where to be found. After moments of elevated heart rate, I grabbed my set of car & miscellaneous keys and started trying those. On the third try I found one key which worked. I believe it was the key to the front screen door at my house but I don't recall for sure. The point is, rather than spend money on the actual Yanmar keys, see if some of your existing keys will work, the get a similar blank for it.
We did find our original key after the engine was started. It had fallen out and slipped through the hole to the area where the emergency tiller is connected.