Don't forget with having an import, that the battery terminals will not be the same size as UK/EU batteries. They have thinner posts. I had to fit post reducers over the battery terminals to step down to fit it to the Jap +ve & -ve clamps. You could change the leads but it is a bit of a faff with the +ve. Watch out for the CCA rating on the replacement battery, as some of the EU batteries are narrower to fit the zed's battery box and terminal posts, and therfore can have a lower CCA than the one you are getting rid of - you'll have a job starting in winter! I use a FISCH F068 with 550CCA and 70Ah from these guys http://www.countybatteryservices.co.uk/delivery and you may be able to get JDM thinner terminal posts fitted when they build it if you ask them.
Back to the ECU: I've not had any warning lights ever, but this auto electrics place in Derby did manage to kill off my JDM ECU when they were fitting the security devices, and it wouldn't start back up, though they sent the ECU away somewhere and fixed the problem after a week (a couple of years back): http://www.mobilesolutions.uk.com/services They might give you their contact, whoever the other company was.