For anyone faced with a similar issue, I thought I'd write up how I fixed this. New front ABS sensors are available from Toyota but are still quite pricey - £200 or so in the UK. Secondhand parts are one option, but parts for the AT200 are quite rare it seems, and you play the longevity game with any secondhand part thats now knocking on 20 years old.
The option I went for was to source a brand new Intercambio Cambiare VE701925 aftermarket ABS sensor for a 1994 - 1998 Toyota Camry. This cost me £30 including postage.
I jacked the Celica up, removed the front wheel, put and axle stand in and wedged the removed wheel under the sill in case the worst happened. Then turned the steering wheel to full right lock, making getting at the ABS sensor, which lives on the rear of the hub easier. The sensor is held into the hole in the hub with a single 10mm bolt.
You can see the sensor unbolted and moved aside here. Next up was to compare the old and new sensor ends side bty side. You can see that the new sensor is identical, but for the fact that the wires exit the sensor pointing 'down' not 'up'. This isn't an issue as we'll see later.
Next, we need to compare the othe end of the wiring - the bit that connects at the top of the wheelarch under the wheelarch liner to the loom that runs to the ABS ECU. Removing the wheelarch liner is fiddly, you need to undo multiple 10mm bolts and at least three philips head screws to bend it away enough to get access to the sensor wiring at the top. I chose to just bend the liner out of the way, but to do it properly undo all the bolts and screws and completely remove the liner. You don't have to though.
The original sensor wiring goes across the outside of the hub, where the damper connects, then goes into a metal bracket/guide thats held to the inner wing with 2 more 10mm bolts. Undo them, and the guide comes away. The connector at the top is a push-in affair with 2 spade connectors
The Camry connector is different - instead of a female connection with male spade connectors, its the inverse - a male connector with female spade terminals
This won't connect to the ECU loom at the top of the wheelarch, so here is where it gets interesting. I decided I had nothing to lose, and as both sensors had 2 wires - one white and one black - I would see if I could cut and solder the old sensor connector to the new sensor wiring. Here is what the top of the old sensor wiring looks like, you can also see the metal guide it sits in
I went to the shed, and carefully cut the wiring so i had about 2" of wire to play with after the connector
I then carefully cut away the black rubber coating that wraps the white and black wiring and holds it in place, and use some wire strippers to expose the bare copper ends of wire so I had something to solder. I then did the same with the new sensor, cutting the incorrect connector off the end, stripping the ends of the white and black wires and then slid a 2" section of heat shrink onto the wires out of the way. Then carefully soldered the white to the white wire. I'm not awfully good at soldering so its not especially neat, but was definitely joined
Next, I soldered the black wire, then carefully slid the heat shrink over the exposed soldered joint, and used a butane lighter to warm up the heat shrink so it it, ummm, shrunk.
Its not especially neat, but had sealed the solder and metal ends so they'd not get damaged or short when splashed with water on a wet day out in the car (I hoped).
The next thing to do was to carefully cut the old cable ties that held the rest of the old sensor into the metal bracket/guide, pull the old wiring out and then use new cable ties to route the new sensor wiring in. The guide is important as it stops the wiring from tangling in moving parts and from chafing. With this done, I trial fitted the bracket into the inner wing and had a look at the cable to make sure it had enough slack to allow it to move with the steering
There is enough slack to allow movement, but the routing isn't ideal. I needed to make sure that the wiring at the sensor end wasn't going to snag in the CV boot/gaiter really on full left lock. Before I did this though, I needed to test to see if the ABS light had gone out. I connected up the connector at the top of the inner arch after giving it come contact cleaner for good measure, put the car into diagnostic mode by pulling the jumper pin from the diagnostic socket, bridging E1 and TC with a paperclip and turning on the ignition. After tapping the brake pedal rapidly for 3 seconds, the ABS light settled into a on-off-on-off flash which indicated it was working correctly on all 4 sensors. What a relief!
I then went back and did up all the bolts holding the bracket.guide into place, refitted the wheelarch liner, and tightened up the bolt that holds the sensor into the hub after cleaning the hole it fit in up with a rough file. Next up was working out how to keep the end of the sensor wiring out of the CV boot gaiter. I reused the c-shaped plastic clip that routes the cable around the bottom of the damper, over the top of the hub, then using three cable ties around the track rod end I was able to make a loop that was loose enough to allow the cable to move with the steering (this is very important) but held tight enough to the TRE to keep it from snagging in any moving parts. I had to trim a 2" section of outer sheathing from the cable where it went through the plastic clip, the cable has a triple layer of rubberised sheath on it, and was slightly too fat to allow the clip to snap closed around it. With the outermost layer removed it fitted fine though.