Important things to think of in a marine application are you do not want a highly stressed car engine in a boat as a car will never go for an hour at full power where as a boat will. So a low stressed big engine works wonders.

Make sure you have either a keel cooler or a heat exchanger as putting raw water cooling through a car engine can result in a dead engine pretty damn quick.

Once you have your engine connected to jet drive be careful when running it as there is a good chance that the bearings are water lubricated and cooled so stick a hose pipe up its arse to cool it.

On engine choice what about a bike engine small damn light with built in gearbox to get right speed for jet but possibly too high stressed. Or a diesel which uses less fuel and you can use pink diesel and since it needs less fuel can actually work out lighter than a petrol, also none of those nasty electrical things to go wrong.

One cool thing about jets is since they have a bucket door for reverse instead of a gearbox you can go to zero thrust with bucket door half way down then give full throttle go no where then open the bucket and scream off with full thrust at zero boat speed.