Dead easy but I can't draw it out here.
Most alternators have 3 terminals. The BIG one is obvious and goes to
battery +.
The sense terminal also goes to battery +. The two are seperate so that
the sense senses battery voltage regardless of the volt drop on the big
wire. However, for a tractor, they can be simply comnnected at the
aternator.
The ign terminal connects to the ignition through a warning light,
typically 2.2W.
You will need to change the battery to negative earth. The ignition
coil and the bulbs will need changing but the 6V starter will be OK. It
will crank quickly but won't overheat unless you are stupid and crank
for ages. Glow plugs will also need changing if it's diesel. If the
thing is 6V, it's so old there's not much more to worry about.
My Ferguson T20 is converted to 12V and there's no problem.

John