sorry, no insult meant. Just be aware that the 6 amp alternator is pretty small for charging electrical requirements on the boat and you are probably better off to fix the alternator belt and use the diesel as a genset if nothing else. (assuming that the diesel alternator is approx 30 amp output). The big advantage of the outboard is that you can always hand start assuming you have an electric start outboard. Assuming you have a seperate start battery and house battery then your house will be approx 80-100amp hours. If discharging to less than 50% then you have 40-50 amphours required to charge your battery. This will take approx 8-10 hours of engine outboard run time but only 1-2 hours diesel run time. While sailing at night your power load will be approx 5 amp minimum (ie 4x15watt navigation light globes require 5 amps) plus radio etc. so you will have to keep an eye on your battery use. While cruising and at anchor you can pretty quickly discharge your batteries.
Also back to the original suggestion, split the chain and fit your spare belt. The chain will have a split link. Chain is not sold as an endless loop. All chain is sold as a single length with a split link so that you can make up the loop to suit your requirements. Remember your bike chain? Find the split link on the diesel chain and go from there.

Don't like the sound of that. If that diesel is still operational I think it would be worth fixing up the smaller issues of alternator etc and giving it some decent use before ripping it out. You might learn of it's true value and get to like it.

Those old exposed flywheel diesels are usually as reliable as the day is long. Outboards are fairly good these days but I don't think they are 30 years good.

Frant is right on the mark re trim and centre of gravity.

+1 for the diesel inboard, I don't have one but I wish I did instead of the outboard.