Early ones were positive earth, later ones were negative earth. Very early ones were 6 volt, but I think only TVO models.
Use a set of jump leads and power up the starter motor while holding the gear selector in S, if the starter spins correctly and the engine cranks when positive is on the starter motor then it is negative earth, if the starter motor spins but the bendix is chucked from the flywheel and it doesn’t crank then its positive earth.
Diagram below assumes there is an amp clock, if not then join those wires together and insulate. Also that is the basic circuit assuming no lights, etc fitted. There is no ignition switch.