I have popped the full service manual I referenced up here for you to download
http://spafrost.me/nissan-silvia-s12...s12_manual.pdf - it should contain the correct sections
I have my full starter circuit bypassed, as the wiring went bad, so I should not imagine it will stop it from running.
The ECU is a pretty simple old thing, as far as computers go - They usually give up completely and you loose spark/injection all together.
When you attempt to start the engine, how exactly does it behave -does it start, run for a few seconds, then die - fire a few times but die if you let go of the key etc ?