Why not a 'precision' rebuilt A4
One that is fully rebuilt and includes all the modernizations such as electronic ignition, electric fuel pump, fresh water cooled. Will weigh much less than a diesel, will be a LOT smoother, will require NO bed changes, NO prop changes, NO fuel tank changes, NO control panel changes, etc. etc. etc. etc. .... and the *total cost* will be reasonable. The best A4 rebuilder probably on the planet is: www.moyermarine.com