I doubt you'll find a cheap engine, but inboard jets are still made and available. Whether you can find one that fits your (French?) motor mounts is another story. Mercury makes an inboard jet right now but its only available as a 200hp. Its possible that you could keep the existing jet and just put in a new block. Adapting your jet to a new block and getting the motor mounts all lined up is probably going to take some work. Try some of your local riverboat dealers, they are pretty jet savy and might be able to suggest some alternative blocks for your jet.