That tractor was originally POSITIVE ground and since most all alternators require Negative ground, you need to make a few changes.
1) The wires on the ammeter need swapped side to side otherwise it will read bass ackwards.
2) Of course, install the battery with its NEG post to ground and Pos is the big cable that leads down to the starter.
3) That alternators output wires to the Load (NOT battery/starter) side of the ammeter and Id upgrade that to at least a 10 Gauge. The ammeters other SUPPLY terminal is what wires to the starter post (I like that 10 gauge also). The ammeters Load terminal may also feed hot battery voltage to the lights and ignition switch BAT input if those loads werent previously fed from an L terminal if it had a 4 terminal Voltage Regulator. The same can be accomplished if you just wire the alternators output to the wire or wires that hooked to BAT and LOAD on the VR, but Id prefer you upgrade to a new 10 gauge and run the alternators output up to the Load (NOT battery/starter) side of the ammeter. If it had a 4 terminal VR with a LOAD terminal, that wire is what feeds hot battery voltage to lights and ignition, but with no VR anymore it could be ran to either the Load side of the ammeter or just hooked right there onto the alternators output.
4) The coil wires need swapped if you convert from Pos to Neg ground for best performance