I broke the key for my Yanmar 12 YSB and Marine Power said they could not get anymore as they were so old. I used a "plant" key, as used for plant machinery. I used a Drimmel with a fine cutting disc to widen the slots slightly and this works fine.
Lost the key to my Volvo Penta last week. I dismantled the lock and took out the levers, reassembled. Now anything will start it. The electrics are normally isolated below so its a minor security issue. Saved forkin out for a new VP igno switch at a crazy price. Probably work for Yanmar too.
Got 2 spares for my YSB8 about 2 years ago from MARINE POWER They are not all the same as the first 2 they sent me were the wrong ones as my instrument panel is from a YSM but the engine is a YSB no problems getting them changed though.
Give them a call they are very helpful.