I have no experience of what you are suggesting but have dealt with them a number of years ago in refurbishing an old jet drive speedboat. They were excellent to deal with and extremely helpful. The man to speak to there if he is still around is Mike Bellamy.
Mike is still there and his advice is as good as ever. I was there this week. I imagine that they have more experience of swaping petrols for diesels than anybody else in the country and I don't think you will go far wrong with Lancing Marine.
You are going to end up with a slower, heavier boat, but one that you can afford to run! You'll be able to get fuel just about anywhere, it will be safer and you'll have a nice new engine.
Look carefully at the economics of the operation. It's only really worth the trouble and expense on a boat you want to hang on to for a good while.