The rectifier BTW is ~£77, kit is ~£133
That doesn't include any charge controller.
From what I can see from googling ( https://www.tohatsuoutboardparts.com...0221054-1).pdf
) you already have the alternator. You probably have a white and yellow wire that terminate at loose terminals. Alternator spec seems hard to find but looks like it would be 60-80W. About 5amps.
If you ran that into this: New 150-250 CC Engine Motor Voltage Regulator Rectifier 4Pin Female Plug | eBay
Yellow - Connect to the yellow wire from the alternator
Red - Connect to 12V +ve of battery via a 3Amp fuse
Pink - Connect to the white wire from the alternator
Green - Earth Connect to engine body, and to 12V -ve of battery
Before connecting to a battery I'd suggest running and checking the voltage with a meter - expect 14.4V or thereabouts.
Secure the rectifier to something big and "metally" that will dissipate heat.
That unit is a rectifier (converts AC to DC and a regulator converts varying voltage to c12V. ) It doesn't manage the battery charging. There is a possibility you could over charge battery if you don't have any drain on the current ever running such as lights or radio etc. If thats a possibility you want a charge controller like you put on a solar panel too. BUT assuming you will have some drain running you will probably be fine, especially as you wont be motoring for 72 hours solid.
If I'm remembering right you said you have 34Ah of battery on board. That would discharge down to about 14Ah and then be 'dead'. Recharging that 20Ah drop will take about 7 hours with no draws on current. But assuming you head out with full batteries and have engine running while the electrics are on chances are you aren't draining anything... its the drifting that will discharge you. You'd need to know what your current draw is in "drift mode" and how long you drift vs motor. To be honest you need the drift mode current draw no matter what solution you adopt or its all guess work...