Yea I don't really know anything about the 300 model. The manual I pulled the info from was for 2wd and 4x4, saw that right I made the post. FW is just the 4x4 version, TRX300 was the 2wd version. Same wire diagrams per year it seems. Didn't know you was in Canada (we don't really see the reflectors down here). Most models should be the same for wiring though I'd think. Engine looks pretty clean. I see there's a bit of a wire mess in the front, but the main harness looks alright.
The ATC250SX harness is pretty simple. Green is ground, black/white is the kill wire. Keep them separated to run, short together to kill. Kill switch on the handle bars would work fine for the switch. To get lights working, you jump the red & black wires where the ignition switch plugs in (2 or 3 pin connector, pretty sure it's 3).
If you get a dash, I modify Honda switches that are still available for the atc250sx machine using the proper plug etc so it's plug and play. If I recall correctly, the wire is slightly longer than OEM. Also sell the ignition key rubber covers since the only way to get them is by buying the whole switch assembly. I take them off the 2nd/spare key to resell. I have something like 50 or so of those rubber key covers on hand if you know anyone into collecting NOS parts w\o orig packaging.