Try and pick the brains of someone who has done it before, the organisers may be able to help. Whilst you are at it, see what they mean by show room standard. I run a RS5F31V in my 1300 Micra which has much better ratios than standard. They weren't available in the UK as an option as far as I am aware but were fitted in other countries. A long shot and they are rare anyway.
It won't solve your ratio question, but the later 1000cc boxes had a bigger clutch which my be an advantage.
Depending on what you are allowed I would look at the Micra Challenge suspension with adjustable camber that Matt Humphries has available. It makes a world of difference and improves the handling.
Given your location it may be worth contacting Ken Wood. He ran Micra for a while and has alot of rally experience (Scottish Rally Champion more than once and last I heard runs a Dolomite Sprint).
