It's the dynamo that needs to be polarised correctly to charge properly. It will almost certainly be negative earth. I would connect it negative earth anyway, if it dosn't charge the dynamo can easily be polarised for negative earth. Just connect the body to the negative terminal of a battery and flash the positive to the field winding. I personaly would replace the dynamo with an alternator, simpler wiring, easier and cheaper to replace if it goes wrong.
At that vintage, the dynamo will almost certainly have originally been +ve earth. As suggested below, the dynamo can be either polarity and it is not difficult to reverse it if needed. When all cabled up, press the “cut-out” contacts together (if you remove the fan belt first, the dynamo may well run happily as a motor) this will set-up some residual magnetism in the field winding core and from then on it will be the correct polarity. This assumes the combined cut-out & regulator are Electro-mechanical (big coiled relays) and not semi-conductor versions. The starter is not battery polarity sensitive.
However, since it sounds as if you are starting from a new-ish install, an alternator from a scrap-yard will cost very little and can easily be fitted with almost no modifications other than a new fan belt required. This will give you over 35 amps charge, compared with 20 max for a dynamo, regulation will be better and the batteries will last longer.
Email me if you need more concise electrical directions for the Perkins as I have done this dynamo-for-alternator swap myself, including having to reverse the polarity of the entire boat.
No, it’s all to do with “shunt” or “series” wound motors:-
Rotation depends on the magnetic flux induced by the DC voltage initially in the field and then the armature windings, which are fed via the commutator and brushes. In a shunt wound unit, reversing the polarity will reverse rotation, but series will always go the same way irrespective, hence your B&Decker drill works the same OK off AC or DC volts and actually requires quite complex switching to reverse the rotation. The starter motor is series wound and hence doesn’t care about polarity.
Or is it the other way round? – It’s been a long time!