Well, the stop circuit may not be working correctly….You could push the black/yellow powerpack lead out of its rubber connector, then ground it while the engine is running. Be careful though, that black/yellow lead will zap you.
If the engine stops OK when the powerpack is manually grounded, you will have to troubleshoot the stop circuit wiring/key switch, etc…
If the engine still fails to stop when the black/yellow lead is manually grounded, you will need to replace the powerpack. I am "asssssuming" the powerpack is grounded properly though.

The powerpack is mounted on the exhaust manifold, next to the cylinder head, it is rectangular in shape. There are two main connections to the pack, one four pin connector, and one three pin connector…The three pin connector has two orange leads, power to the coil primaries, and one black/yellow lead, which is the "stop lead". The key switch grounds the black/yellow lead to kill the spark, which simply drains the capacitor inside the powerpack.
Be very careful handling these rubber connectors, please do not simply cut the black/yellow lead in order to isolate it. Carefully separate the three wire rubber connector. The engine side of this connector might be easier to deal with. Do not spray oil/WD40 into the connector in an effort to ease pin removal, soak the connector in alcohol. Spray lubricants can cause a short between the leads, which will lead to problems down the road. There are special tools to remove and install those pins and sockets, I realize you don’t have these tools. Decide whether you want to push out the black lead pin in the engine side of the connector, or the black yellow lead socket on the pack end of the connector. Perhaps a small nail will fit into the black/yellow lead socket, making it the easier choice for removal. After soaking the rubber connector in alcohol, push the socket out of the rubber connector being careful not to jab yourself doing it. Once the black/yellow socket/lead is out of the connector, reconnect the connectors making sure the black yellow lead is not touching ground. Start the engine, then use a pair of insulated needle nose pliers to ground the black/yellow lead on a good clean ground, the engine should shut right off. The powerpack will need to be replaced if the engine does not shut off immediately.
If the engine does shut right off, we will have to troubleshoot the stop lead/circuit in the engine harness/boat wiring….D