Repowering a sailboat is an easy project as long as you can remove the old engine and drop the new one without having to disassemble them..
The main issues are going to be:
Engine brand/model and Reduction gear choice: Here you have to do your research and due diligence.
Once you have removed the old engine and presented the new one in its place, you will be able to tell what if any changes to the engine bed need to be made. Also the lenght of the shaft could be determined. If a new shaft is needed then haul out and replace the bearing (if showing wear) the shaft and the prop.
Once the new shaft is in position with the coupler installed, position the engine on its bed and align port/stbd, mark and bolt mounts. Stop the aligment work at this stage and install s/w, f.o. and
exhaust hoses and the wiring
harness/gauge panel. Lauch the boat, let it settle and complete the aligment of the engine/trani to the shaft coupler. Seatrial.
You can reuse your tank (flush it), tie the return to the vent hose.
All the details involved in steps above are all over your new engine owner's manual and the internet.
If you decide to go with a diesel engine then the choices will be a used engine, rebuilt engine or a new engine.
The main reason people re-power is beacuse they have had enough of the problems i.e. lack of reliability.... with the cost of labor being what it is put your $ to the best use and get a new engine. i.e. 00000 hours, no problems, 100% reliable. Regardless, you will be having to deal with installation whether you choose a used/rebuilt or new engine.
...George.