Sterngear
Unless you're going for very high power outputs, the stern gear from the 145HP Perkins will be more than sufficient. The shaft; coupling; stern-gland; p-bracket and propellor should all be retained if they are in good condition and refurbished if not.
The prop-shaft is usually 1 1/4" (31.75mm) in diameter. For very high power outputs, a change to a stronger material in the same diameter will cope with the power without the need to change the stern-gland and p-bracket. failing that a move to 1 3/8" diameter shaft may be necessary, but you'll need to change the stern gland, p-bracket cutlass bearing and maybe the propeller hub taper.
Gearbox
Typically the Fairey Power boats used a Borg-Warner 72C (and occasionally 72CR) gearbox. These are good for just about all the power options on the Cummins, but might need refurbishment to make sure they can cope.
Engine Bits
From the motor itself, the gearbox oil cooler should be retained along with the gearbox adaptor plate.
Ancilliaries
The alternator can also be re-purposed on the new motor if you were not luck enough to obtain one with your salvaged engine.
Although the Starter motor will fit the new engine (with a change of pinion) we've found that typically, it is not man enough to start the Cummins.
Turbocharger
If the Perkins has been fitted with a later Holset H1C turbo-charger, this should be retained - and refurbished if necessary - for motors up to 23oHP.
The higher power engines require the water-cooled turbo-charger.
The H1C is easily and cheaply refurbished as spares are readily available. The earlier 3LD and Schwitzer derivatives are very costly to maintain so can be discarded or sold on.