Silvertop 4age not running

[davev]

Hey,
So I have an ae92 fxgt with a silvertop in it. I was driving along the motorway the other day and it started misfiring, then the cat overheat light came on. When I applied throttle it started to decelarate. I put it in neutral and it stalled.
After pushing it off the offramp, I had a look and the ignition lead to cylinder 3 had broken in two.
I got it towed as it wouldn't start and have been trying to get it go the last two days.
I have replaced the leads and plugs which were buggered. Still no start. I got codes 12 and 21 on the diagnostics. I reset the ecu and tried starting again. Then only code 21 showed up. There is spark and fuel and the car wants to start every so often but doesn't quite get there. We compression tested it and it is a little low on 3 but nothing that should keep it from starting.
We couldn't find our timing light so couldn't check that. But it seems unlikely that the timing would be out considering the dizzy looks fine and is tight on.
I am going to get another o2 sensor (due to code 21), does anyone know what the problem could be or has anyone had this problem before?
Cheers

[CXGPWR]

Had this problem once with a customer that thought it would be good to change his spark plugs but was too cheap to put platnum or iridium in it. Standard spark plugs even new it didnt like to start. put platnum or iridium in and bang, started first pop. Could be a go er? otherwise check air flow meter & no air leaks between said meter and engine

[Mr Revhead]

Not having $$ plugs won't make it not start. There is another issue there.
Whats the conversion like? Dodgy or tidy? Start looking for loose wires!